I've been using the Sensei since about October. I got it for maybe $64 shipped cause of the discount they gave out at Blizzcon. I really like it. I've used a Deathadder, Orochi, Imperator, G9x, and 518, and It's probably the best mouse I've ever used. Sure, it's a laser mouse, but it tracks great for what it is on my cloth pad, so take that however you wish.
Also, I'd like to touch on Steelseries' support. The first Sensei I was using had an issue with the middle click where it would double-click after pressing it once. It started to really bug me, so I sent it in about a week ago. Today, I got a package in the mail: a BNIB Sensei that works perfectly. And since I had already been using their clunky software on my PC, I didn't even need to set up the mouse again—my settings just transferred right over.
Awesome mouse, awesome company. I'm happy, so if you're willing to drop the cash, I highly recommend the product and the company.