I use rollers 3.5 inch aluminum cycleops, they are great all the way through the gears. Yes it is true that the resistance is not the same as on the road, that is not their intended purpose...

The purpose is to...
1. Work on form, including ability to hold a line
2. Base Training, lower HR and resistance
3. Feel natural, like being on the road.

They are great for sprint training because you can do spin ups in a really high gear and simulate the action required. Now you can not standup and hammer away but they force you to be smooth at really high (over 130) RPMs and that translates to better sprints. I like them to warm up also because I can use the race wheels on them (no major tire wear) and I get into the feel of riding. Fluid, magnetic and air resistance trainers just feel well unnatural.

Trainers are great for environments in which you need full resistance of the road for advanced training (i.e. intervals, full intensity sprints). They are also good for longer rides because you do not have to worry so much about always being in perfect form... good or bad. Rollers take alot of attention and riding inside attention after about 60 minutes goes away and you fall.