These are the are the ranges as I see it:
<4ft/s: Really, really slow
4-6ft/s: Medium slow, good torque, very good controllability
6-9ft/s: Medium speed, torque, controllability
9-10ft/s: Medium fast, decent torque, decent controllability
10-12ft/s: Fast, somewhat low torque, controllability depends on the drive design and driver
12-14ft/s: Very fast, low toque, probably needs a good driver
14-16ft/s: Extremely fast, very little torque, edge of controllability, needs a skilled driver
16+ft/s: Usually too fast. Very few teams have ventured into this territory except as the high gear of a shifter (254/968 come to mind) and I can't think of a successful single speed robot going this fast. Requires a quite good driver.
Of course, speed should be determined on a case by case basis; for example, in 2006 if your robot design was like 25's you could probably get by going 16ft/s even with a below average driver. This is because the turreted shooter only required that you get near the goal and you didn't have to maneuver finely to acquire balls. On the flip side, 2005's game required a lot of fine positioning so a low speed would probably be better. Also playing into the equation is your amount of traction and number of motors, but I wrote about that
here so I'm not going to write it again.