I don't currently have one installed, however I do have a friend that has installed one and he absoulutely LOVES it. Both me and my other buddy are going to install them on our sleds before next winter (700 rmk and 600 xc). Everything I've heard has been positive.
I have a polaris 800 and a skidoo 670. I have it installed on the 670 and it works great. I live at 3000' and go to the Black Hills where it is 6000' and it has worked great. I've put on about 1000 miles with no problems.
The 2000 Polaris RMK (Trail) of mine has the "ACCS" (altitude compensation carburetion system) factory installed. The manual say's - carburetor jets do not have to be changed with a change in altitude.