As an iPhone 4 user since last year I had the chance to try out my first Android phone, the current top of the line Samsung Galaxy S II (i9100) thanks to TRND. COM
And I have to say I am really impressed with what I saw and experienced with this Samsung phone.

---- Hardware (general build and feel) ----
Extremely lightweight, very nicely balanced. The handset size seems very good (perfect for a man's hand although my wife also likes it very much), actually hides its actual size due to its thinness and weight.
Materials used have a good haptic touch, although it feels slightly less expensive then the iPhone 4 (probably because it uses more plastic and less glass/ metal).
The back cover is slightly difficult to remove because it initially feels flimsier then it actually is but unproblematic once you get used to it and no problem at all if you need to insert a microSD card, replace the SIM or change the battery.
A very high quality in-ear headset is included which I massively prefer instead of the Apple headsets - but this may be a personal decision.
-> Conclusion: In all the Samsung is on par with the iPhone 4, each has its own merits.

---- Screen ----
Large (4.3"), very crisp and sharp display, great colors (better then the iPhone4 Retina display in this regard but also a bit darker).
The screen (using an enhanced AMOLED technology which Samsung coined "Super AMOLED Plus" is probably the best stand-out feature of the Samsung and puts it over the top of all current smartphones.
-> Conclusion: In all slightly better then the iPhone 4 due to size and colours.

---- Operating System ----
Uses Andoid 2.3.3 (named Gingerbread), the most current smartphone version of Google's Android OS. With many great and useful Android features (notification), but others like Apple are catching up fast to we need continued progress. And this is good for the consumer!
I had no crashes or reboots using the Samsung during the last four weeks, only once the Facebook app needed to be restarted but I guess that was the app itself and not the OS.
-> Conclusion: Probably just as good as IOS 4, I prefer the Apple although this may very much depend on the personal preference so I will call it a draw. (3:2)

---- Samsung OS Add-ins ----
Samsung added its own refinement ("TouchWiz 4.0") on top of Gingerbread to enhance the experience.
I loved the fast access to settings like turning on/off Bluetooth, GPS, WiFi etc and some scalable widgets seems useful.
I am not too sure about the hubs concept - I personally rather like to use specific or native apps for specific media types.
Samsung also included a DLNA compliant server application, which enables the phone to wirelessly "send" pictures, videos and music from the handset to a number of devices like modern TVs, Playstation 3, Xbox360 or even to a Windows 7 PC.
I personally like the optional Swype keyboard which Samsung provides - this enables you to type words without lifting your fingers. After using it once on the Galaxy, I missed such a feature on the iPhone.
The Samsung also has a remote management feature ("Samsung Dive"), which enables finding or disabling the phone if it lost, similar to the "Find my iPhone" service of Apple.
Samsung provides an own software suite for Windows and OSX PCs called Samsung KIES, which even enables a wireless sync to the PC. Media management of KIES is good but not as good as Apple's iTunes. There are a few sellers for Android music (most prominently Amazon) but Video content does not seem to be available anywhere.
-> Conclusion: Better then IOS 4, although this may really depend on the personal preference, especially if do not like to "bring your own media".

---- Speed ----
The Samsung has a 1.2GHz dual core processor and it shows in general responsiveness - I never had the feeling of lag while using the touchscreen or apps. Of course measuring and comparing speed within apps is difficult (for example Angry Birds game seemed slower - lower FPS - then on the iPhone4) but the web load rendering on WiFi and 3G is noticeably faster than the current iPhone generation. Also a reloading of web pages was rarely required, possibly also be a due to increased memory (1GB RAM). The problem with speed within graphic intense games may be a general issue with Android OS, not anything Samsung can directly improve.
-> Conclusion: Slight preference of Samsung, but YMMV because this may depend on the apps used. Will probably be no difference for most users.

---- Battery ----
My own requirements are that any phone should last at least 36-40 hours with my normal usage pattern. 40 hours would be perfect. Take a fully charged phone from the bedside, use it normally during the day and only require charging again the evening of the NEXT day, when you go to sleep. This way you know you'll have enough power if (i) you could not charge it daily and (ii) required more usage then normally (e.g. on a trip when you need to use GPS and 3G more frequently then usual).
Until now the only current generation smartphone capable of that was the iPhone4. Now the Samsung got this right, it gets me through a whole day and a half. Although my iPhone (even with it's almost year old battery) is slightly better, above the 10% charge around the 40 hour mark with own usage, I did not get the chance to "condition" the Samsung battery as I regularly do with my iPhone. So your mileage may vary. And of course you can carry a swappable replacement Samsung battery (reasonable cheap - around 20 on Amazon), which is not possible for an iPhone at all.
-> Conclusion: Very slightly prefer the iPhone, but the Samsung would be ahead if I took a spare battery into account.

---- Camera ----
Until now I believed the best smartphone camera was on the iPhone4 but Samsung definitely beat Apple's phone. Each and every picture I took (outdoor, indoor, low-light) was better on the Samsung Galaxy S II, even if I compare the front cameras of both phones. And the HD video capabilities also seem to better then Apples top model. I only miss a build in HDR functionality (even the pseudo HDR Apple delivers would have been good) but this can be remedied using a number of camera apps from the Android market. And a dedicated camera button would have been great but the
-> Conclusion: Samsung first, iPhone4 is clearly only second best, and other phones have al lot of catching up to do.

---- Connectivity ----
It has all the current features (3G/HSPA+, WiFi, FM-Radio, HDMI & USB) but what really is a standout feature for me personally is the Bluetooth rSAP support (Remote SIM Access Protocol) as part of its package of Bluetooth protocols. This enables my build-in car phone to connect to the Samsung, something no iPhone and barely any other smartphone can do!
The Galaxy S II version for the German market does not have 4G but it would be useless anyway with its current rollout status here.
-> Conclusion: Samsung wins, at least with FM Radio & HDM, even if people do not care about rSAP Bluetooth as I do.

---- Future OS Upgrade ----
Not known, currently has 2.3.3 (Gingerbread), any newer version (e.g. Ice-cream Sandwich) will depend on (a) your phone provider (b) Samsung and (c) Google. But most manufactures promised to update at least for 18 months and then there is always the rooting community. But since Apple has provided around 30month of firmware updates to its prior iPhones the 18 month do sound only very basic.
-> Conclusion: Here Apple has set a benchmark to beat (firmware support up to 3 years), Samsung has to prove it can deliver on its promise.

---- Price ----
The price may depends of the type of contract you have with your provider but if I take the unlocked and no contract version into account, the Samsung is very reasonable and much better then its direct competitor. The street price (e.g. Amazon) six weeks after it's launch is around 470 GBR, an iPhone 4 still costs at least 510 GBR one year after it's launch.
But do not forget that the one time cost of a mobile is usually nothing compared to the total you are paying monthly for the 2-year contract.
-> Conclusion: The Samsung Galaxy S II is the better deal ("more bang for the buck"), with or without a contract.

--------------------------
---- Final Conclusion ----
Definitely the best Android mobile phone, it wins or is at least on par with all the key factors. My (subjective) comparison would give it an 8:6 lead against an iPhone 4.
The Samsung Galaxy S2 has no real weakness, very much unlike most its competitors.
For any "new comer", current Android users as well as many iPhone users it should be a no-brainer to choose the Samsung Galaxy S II.
And I would have probably switched to this Samsung without hesitation but at the moment I would need to justify the loss of all the investment I made to the Apple ecosystem (apps, accessories, media).

---- What Samsung should improve? ----
- Documentation: There was not much, especially more info about Samsung's own apps and pointers towards the KIES Windows/Mac software would have been great.
- Branding: Is it called Samsung Galaxy S II or Samsung Galaxy S2 or Samsung i9100? Just own your brand and decide on ONE name with one spelling, worldwide! Especially do not allow the carriers/providers to change the name (as it is often done in the US).
- Camera: An external hardware shutter button would be nice, as would be build-in additional camera app features (like HDR or filters).
- Updates: Apples keeps it iPhones up-to-date with new Firmware for about 30month (at least that what they did with the first two generations). It would be a very good sign for the consumers if Samsung did the same or at least provide updates to the customers for more than the 18 month they recently announced together with Google. After all a contract is usually 24 months for the end customers so this timeframe should be covered!