It is called many things, but there is a delay bug on most mx1000's (mine included). When you pick up the mouse and set it down again (doesn't have to be much, the amount necessary to move it on a mousing surface is good enough for instance) there will be a short delay before tracking works again. For me, this makes gaming impossible with this mouse - which sucks, because while I am not lagged out, I sure frag a lot more than with my mx700's Supposedly this isn't on all mice - my friend for instance bought one even after I warned him about the problem, and reports no such problem after testing. So... it's a risk, I suppose.
Also, the new side buttons make it impossible to press both the back and forward buttons at the same time. This won't affect most people, but it does me, as I use both of those buttons in games and often I have to use both at the same time. The reach on them is also a bit different, if not uncomfortable, compared to older mice.
Also, there is a more greyish unit on the market.
For desktop mousing though, it sure is a nice unit