I use an MX518, that replaced my old 510 (last year! those things last forever!). The main thing I want is a comfortable grip and extra buttons. I usually bind melee and throw grenade type actions to the thumb buttons, which frees up some space around wasd for misc stuff. The small scroll buttons by the scroll wheel are also useful for binding utility stuff that doesn't need to be accessed super fast but you still don't want to reach across the keyboard for. (Like switching to certain weapons, turning laser sights on and off etc.) The mouse wheel I rarely use, since switching weapons with number keys is always faster and more reliable. I experimented a bit with the sensivity switch functionality but it mostly got confusing, and used up buttons that I'd rather use for other stuff.
In conclusion, I don't feel I have much use for all the fancy features of "gaming" mice, I just want me a handful of extra buttons and general high quality. I could probably find a use for even more buttons than the 518 has, but at that point the prices quickly escalate and in the end it's not that important to me. If I find myself with an excess of monetary funds in the future though, I will probably get something fancy anyway.