I have a set of the Kreitler Dyno-Myte rollers and they are very well made. You can get a pretty good workout on them by using different gears, even if you're just riding for half an hour or so (which seems really long when you're just sitting there pedaling in place). They fold up in the middle and you can stow them away in a closet or under the bed.

The one possibly negative thing I noticed about them (and I don't know if this is common to other folks) is that the aluminum rollers leave a trail of silvery deposits on my tires after I use the rollers.

Probably the biggest impact the rollers will have on your riding is to improve your balance on the bike. In the beginning, riding on rollers is like walking on ice in leather shoes (pretty scary because you won't have anything to hold onto in case you lose your balance).

-Kevin