I really like my Microsoft 4000 keyboard. It took a lit bit to get used to it, and I wish the hotkey utility allowed me to create different types of macros than just program launching (certain special characters would be nice), but its a good unit. I initially did not care for the curve. I had a split key that did not have the curve for years from Micro Innovations, and it was a fantastic keyboard that survived 9 years of heavy *abuse*. Its replacement will not survive anywhere near that long.
The downside? Keep it the heck away from water/soda/tea. These are not the most durable things, they are pretty much impossible to take apart, clean, and put back together. If they ever made of these I could put through the dishwasher to keep it clean I would a happy camper. (They are getting closer, but it wont be made my Microsoft when it happens).