I picked up the MX518 this weekend (from Best Buy) and it rocks! It's MSRP there @ $50. I have the MX500 and the MX518. There is a largely noticable performance difference between the two. I like the 3d look to the 518 and it feels slick in my hands (compared to the slightly rough exterior of the 500.) I enjoy the ergonomics of the 500 so the 518 fit my hand like a glove as well. Although they look the same on the bottom, the 518 glides better across surfaces which feels 'just right.' When I first plugged it in, I didn't even install the drivers from the CD or the net. I started to test out the movement in WinXP using the 'on the fly' preloaded settings of the mouse, which are set at 400, 800, and 1600dpi. On 1600dpi, it's somewhat uncontrollable for me. I'm used to moving the mouse a bit so it'll take some getting used to. I installed the drivers/software from the included CD and went to the advanced settings. You can set up to 5 different dpi settings ranging from 400 to 1600 dpi in increments of 50. I loaded up Counter-Strike: Source and went into one of my favorite servers, NBF! I used the 3 default dpi. 400 is great for accuracy. 800 would take a little getting used to for me. 1600 was just too much for me. I played around with the settings for a bit and will probably keep it from 400 to 800 being the highest.
Anyhow, the MX518 is a great gaming mouse so get one if you can!