I have a friend with the MX700, and it works well. Don't notice any mouse lag, even in FPS's.
A note on Microsoft. People can say whatever they want about the company's software, but they are second to none when it comes to service on their hardware. I had my Intellimouse break on me. I generally don't bother with warranties, so I just went back to an older mouse that was laying around. I found out a month later that there was a known issue with the mouse, and that Microsoft was sending anyone who asked for one a replacement with no need to send in the original (just a reciept or a photocopy of the bottom of the mouse). Got mine in two days (I live 30 mins from MS, so that probably helped), and, after taking apart the old mouse and screwing around with its guts, I was able to repair the old one. So, now I have a backup if this one ever fails. Microsoft took responsibility for a design flaw, and that is something that you rarely see anymore. Sorry if that was longwinded, but I was impressed and I am not easily impressed.