I have the X8 and am pretty satisfied with it. I liked my old MX900 better for the form factor and the buttons. I'm not a big fan of the horizontal clicks on the mouse wheel. It's a bit awkward to use because I'm not really used to it. The horizontal scrolling isn't really that useful either because of high desktop resolutions.

The wireless response is really great. Although, I never noticed lag with the MX900 either. The transmitter is a nice upgrade because after a while my MX900 stopped charging in the cradle and I had to start swapping batteries. Battery life is great and needs charging about once a week with 1 AA battery. This is a huge improvement over my MX900 that needed charging every day near its end of life and almost every other day when new with 2 AA's. It also has a power switch that is pretty handy when transporting your mouse.

The X8 doesn't have many buttons that you can program. This is disappointing since I don't need 3 sensitivity adjustment buttons. I barely need 2 settings. I wish those buttons could be programmed. The macro record button is just plain useless but I guess some people like recording stuff for later use. There's supposed to be a way to program a button to do 180 turns but I never figured it out and I don't really care. There's a "Microsoft Game" button or something. It's supposed to open up the Games Explorer in Vista but that worked once. I read some trick to disable it, tried it and it never worked even after trying to reactivate the button. Oh well, I hate the Games Explorer anyway. What a useless button.

I'd probably buy the X8 again knowing this. It's really the closest mouse I could find for comfort since those new wireless Logitech ones are a completely different shape than the good ol' MX series. I just wish there were more mice with better placed programmable buttons.