You can read some user reviews here: http://www.overclock.net/mice/105423...00-review.html
But overall if you've ever used the MX510/MX518 it has a similar body (slightly different texture), it does have a higher DPI (3600) as opposed to the older one and its a pretty solid mouse for regular use. While some people will nitpick on some features like prediction and acceleration but its very light and wont be a big problem. I personally haven't tried the G400 but I have used the MX518 for a long time and it served me very well. I did however move on to the G9x as I am more of a claw type user but my sights are somewhere else.
If the G400 acts anyway like the MX518 its pretty solid on games, did pretty well on shooters and they did improve on the things the MX518 lacked such as response rate. Very responsive, you can map out the buttons with the gaming software, dpi switch on the fly, durability (10mil clicks guaranteed?), doesnt cost as much so thats always a plus