I was a previous iPhone user, until I switched from the 4 to a Galaxy S2, and I loved it, but there were alot of things wrong with the S2, Samsung has listened to user feedback and used their experience to make this 4.8 inch masterpiece. Why do I love this phone so much?
1) Performance - Scrolling is buttery smooth, gaming is buttery smooth, even throwing NOVA 3 at it will not phase that quad-core Exynos 4212 and the improved Mali-400 beast (oh I mean chip) inside this phone. It blows the Tegra 3 and the Snapdragon S4 clean out of the water, sorry One X owners.
2) Audio - The S2 has okay audio quality, it's Yamaha chip was fine for...most people, however it was a bit disappointing compared to even the iPhone's sound quality, so I always used software decoding for listening to music. The S3 has seen Samsung's return to the Wolfson audio chip, and it is absolutely stunning especially with the new SoundAlive system which allows Bass Boosting, Clarity, and Virtualization out of the box.
3) Camera - The S2's camera was good, the S3's camera is even better. It may be a 'measly' 8MP, but Samsung has played around with the software in order to create a camera with zero shutter lag, a definite plus. Now it can take great looking photos, quickly.
4) Battery Life - The GS2 has a notoriously horrible battery life, it was basically essential to root and install custom ROMs because of the battery life. The GS2 had a 1650 mAh battery, the S3 has a 2100 mAh battery. It may have even more power than the S2 but the battery life is definitely improved, I just watched over 2 hours straight of 720p video and the battery meerely went from 85% to 70%. If this was my GS2 it would have gone from 85% to 60 or even lower. Battery life while using data has also been improved, an hour of internet browsing hit my battery only 10% rather than the 20 or 30 that would have been seen on the S2
5) The feel - The S2 was made out of plastic, very light plastic that felt cheap. Of course I know better than to judge how "cheap" a phone is by lightness, but many other reviewers hated it for this very reason. Samsung answered by making the S3 out of poly-carbonate, the same material as a One X. As a result, it feels solid and light in my hand, amazingly it managed to be 5g heavier than the One X while feeling so much lighter.
6) The "new" TouchWiz - I know TouchWiz has had alot, I mean alot of hate over the years. I hated TW3 myself, and didn't enjoy the orignal TW4, only enjoying it's lock screen. The new Touchwiz shipped out with this phone that has a nature feel to it feels great. The ripple effect on the unlock is very enjoyable and I enjoy the Galaxy Nexus inspired app drawer. As a plus I am so grateful to Samsung for removing the horrible Social and Media Hubs.
Of course however...in everything there are things you don't like, and this is no different. The HD Super Amoled screen is great, but, I really wish that for this phone they would have created the Plus version. The Pentile matrix is just inferior to the RBG RBG layout. The ppi is high enough however to stop the picture from being blurry, so its just a nitpik. The things like SmartStay and S-Voice are frankly gimmicks though, and while they seem to have potential they don't really...work. Frankly Iris is better than S-Voice.