I just recently got on my rollers again after using them for the first time last winter. I mostly bought them due to time constraints with a full time job and family, otherwise you can get away with year round riding in Texas. I commented on here how strange it felt with little to no resistance and UnfilteredDregs (where is that guy?) mentioned lowering the psi in your tires to get more resistance. It really works. Put my 32mm tires down to 40 psi and it was enough to put my HR in zone 3 where it really shouldn't have been. Tried 50 psi last night and it was just right for zone 2 spinning with some resistance. You could also change gears.

For sure, if you want structured workouts that require variable resistance, then definitely get resistance. Though at that point, a trainer would probably be better if you could control it from your bars and really concentrate on the wattage without worrying about falling off. But for me, rollers are more about riding form and easy zone 2 workouts in the comfort of my kitchen. I still aim for actual weekend rides, even in the dead of winter. (Pending scheduling restraints. Having a new baby this year has made it tough.)

EDIT: I forgot. I bought Tacx Antares rollers. Don't buy those. They work for me, but are not perfect and took work to make them work properly. If I were going to do it again, I'd spend realy money on Kreitlers and be done with it. Or maybe the nashbar/performance ones with aluminum drums as a budget option.