Here are some more numbers (just for comparison). I wanted to check what my roller (Performance TravelTrac w/ resistance unit) were capable of applying. I already had PowerTap so a test was simple.

These are all approx. because the resistance increases with speed but here are some power numbers. I'll call level 1 the lowest resistance setting possible. I did this test at 20 mph and a cadance of 92 RPM after a 10 minute warmup.

Level 1 - 175W
Level 2 - 240W
Level 3 - 278W
Level 4 - 296W
Level 5 - 336W
Level 6 - 387W

I also varied tire pressure on a few occasions (prior to getting the resistance add-on) and found it to be extremely effective like others have said. Dropping to 75or even 65 psi is a great way to add effort on regular rollers.