I've been using the Razer Diamondback for 1.5 years. I like it so much, I bought 3
This is a love it or hate it mouse; hated by people who are used to palm grips a la Logitech MX500/510. On the other hand, if you can get used to holding the mouse with your finger tips, the Diamondback is THE best. There is not much competition left in this category- Logitech G3, Razer Copperhead are the only real alternatives.
Diamondback vs Copperhead
Pros of DB: more reliable sensor (deal maker), cheaper
Pros of CH: official 500/1000hz support, better side buttons
I bought a DeathAdder this week. It is a great mouse. Basically, it takes the best of Copperhead (500/1000hz polling, onboard memory), Diamondback (newer infrared sensor), and puts it in the most popular mouse shape (Microsoft Explorer 3.0). All in all, I think the DeathAdder is the best palm grip mouse. Check out this page http://www.esreality.com/?a=longpost&id=1300293&page=4
for an objective test of sensors from all the popular mice.