Pros: Solid tactile feedback, reliable tracking, good build quality
Cons: Side buttons a little too high up, no on-the-fly dpi adjustment
The Xai also has an ambidextrous design so people wanting a more ergonomic right-handed shape might not like this too much. My only gripe is the slightly high side button placement, but this is probably a result of me having small hands. It's not terribly high, it just could be a little lower for me. Also the extra side buttons on the other side of the mouse are unable to be programmed to be on-the-fly dpi switches, so that's a downside if you are used to on-the-fly switching. The middle button allows you to switch between profiles which could have different dpi settings but this requires the use of the scroll wheel as well and so can't really be done in the heat of the game.
I haven't gone through any formal tests for acceleration/deceleration but suffice to say I haven't noticed any while gaming. I play a lot of first person shooters where accuracy is paramount, and the Xai has served me wonderfully.