+ G400 has a higher resolution sensor than the MX518, 3600 DPI vs 1800 DPI respectively (I use 800 DPI so it's irrelevant for me though).
+ G400 has a default 1000hz polling rate (1ms response) compared to 125hz for the MX518 (8ms response), thus it is significantly more responsive in terms of figures, but not much as far as the human senses are concerned. It should also be noted that it is possible to force the MX518 to use a higher polling rate by increasing the sampling rate of your USB ports; 250hz shouldn't be a problem, but it couldn't quite manage 500hz in my experience.
+ G400 uses an optical sensor with red LED for illumination which works perfectly on many surfaces such as cloth mats and satin glass; clear glass is still problematic though.
+ G400 doesn't have angle snapping anymore (an algorithm that helps users to move the cursor in a straight line), whereas the early G400 and MX518 did; this is a matter of personal preference, and how much you notice this is dependent on your settings; I've found it to be a very subtle difference, but personally I prefer it without the algorithm.
- G400 has a thinner cable making it more prone to wear and tear. I started experiencing issues with the mouse after 12 months which turned out to be damage to the cable. This is the shortest length of time that a Logitech mouse has lasted me and can almost certainly be attributed to this change. The mouse has now been replaced with a new G400 under warranty as a result.
- G400 shell has less grip than the MX518's which was very problematic for me. I rectified this with some simple DIY (see the detailed section below), but prior to this holding it became very uncomfortable due to needing to squeeze it much harder to ensure that it didn't slip out of my hand.
To sum it up, the G400 is a mouse that technically performs as I require, with some odd design choices regarding the materials used and the cable thickness.
I didn't try out the Logitech drivers so can't comment on them.
I am a keen FPS player and have used the MX5xx series of Logitech mice for the last few years (MX510 for 4 years, MX518 for 3 etc). They are relatively cheap, and while this results in a lack fancy features such as on-board memory for storing profiles for instance, they do provide enough raw performance to rival any other gaming mice. Recently my attention was brought to the G400, which is essentially a continuation of the MX5xx series (in fact the packaging says that it's the new mx518) and I had an Amazon voucher for £20, therefore the mouse would effectively cost me around £6, so I thought why not?
One of the first things I noticed about the mouse was that the cable is thinner than the MX518's, and as my previous mice have all broken due to cable wear I was quite concerned by this. I do my best to look after the cable by taping it to my monitor to ensure it isn't being snagged or bent too much, but unfortunately issues with it cropped up after just 12 months and it was completely unusable one month later. This is by far the shortest period of time that a Logitech mouse has lasted me and in my mind is almost certainly due to the change of cable. For comparison my MX510 lasted for 4 years before any issues and still serves as my mouse at work another 4 years on, and my MX518 managed 3 years before issues started to set in. On another note, my experience with Logitech's warranty replacements has always been positive; I received a new G400 within a few days of sending the old one back. Moving on...
The G400 still uses an optical sensor with a red LED for illumination, but the fact that it sports 3600 DPI is irrelevant to me; I use 800 DPI and can't imagine anyone really needing more than the 1800DPI offered by the MX518 in games if they set their sensitivity up properly, but it's a selling point that mouse manufacturers love spouting and will continue to do.
The mouse does feel very snappy with 1000hz polling rate, but the difference between this and the MX518 at 125hz polling rate is minimal; I have a 120hz display so detecting the difference in response is virtually impossible with my mere human senses, but it might make the difference once in a while. However it is also possible to force the MX518 to use a higher polling rate if you feel that way inclined; it can easily manage 250hz (i.e. a 4ms response time), and also just under 500hz. For Windows there are a few third party programs that will allow you to increase the sampling rate of your USB ports, or you could opt to do it manually if you so wish.
More recent models of the G400 don't have angle snapping, whereas the early G400s did (the MX518 also had this feature). Angle snapping assists with moving your cursor in straight lines, and as such it could be beneficial in games if you are trying to keep your aim level in an FPS for instance (e.g. at head height). The absence of this in more recent models is down to customer request, and so Logitech removed the feature. What this means is that your input is more accurately reflected now, but you will need to be more precise in your own movements to keep it steady. I have tried both versions of the mouse and have to say I prefer it without the angle snapping when playing FPS games. This is certainly personal preference, but it would have been nice to have the option to turn it on or off.
Unfortunately the shell of the G400 has substantially less grip than the MX518's, and due to the way I hold the mouse (picking it up between my thumb and little finger with my other fingers resting on the buttons on top) I found I was having to squeeze the mouse much harder to maintain a reliable grip, and this quickly got really uncomfortable. In some cases, keeping the left mouse button pressed whilst picking up the mouse was causing it to slip out of my hand. This may be partially due to my small hand size and grip style though, as other players have reported no problems with gripping the mouse.
All was not lost though, I had a plan!
Since the shape of the G400 and MX518 are identical, I decided I to exchange the shell of the G400 with that of an old MX518's. The screws on the G400 are hidden behind the mouse feet which have a good chance of getting damaged during removal, so I ordered some replacement feet just in case. When these arrived, I carefully took both mice apart and swapped the shells. I decided to keep the matte grey bit from the G400 and put it on the MX518 grip, put it all together and fitted the new mouse feet on the bottom. It's fairly straightforward to do but it's a hassle I could have done without. This worked wonderfully for me, but I find it bizarre that Logitech changed the material of the shell in the first place.
The G400 is a great mouse that I love playing fast paced FPS games with; I feel I can rely on it to perform my intended actions responsively and precisely every time. It's just a shame that Logitech decided to go for a thinner cable and use a different material for the the shell; these have both been issues for me since getting the mouse unfortunately. Generally the mouse is very similar to the MX518 with exactly the same shape, and so if you have one of those already I would recommend that you stick with it rather than "upgrading" as there really isn't much to differentiate the two mice in terms of performance.
I hope you find this review useful. :)