Big ups on the rollers. I scored some Kreitlers on eBay for about half of their retail price a while back. They were coleslawesome all winter.

First things first, you cannot ride off your rollers and suddenly smack into a wall at 25 mph. If you ride off, you ride off to one side or the other and just kind of slowly fall over. I ride in a doorframe so that, on the off chance that I start to tip over slightly, I can just prop myself up with my elbows. That having been said, I haven't ridden off them in a while.

Second, they're not really that much bigger than a trainer if at all. They fold up real small (i.e. the diameter of the roller tubes x less than half the length of your bike) and when unfolded they have the same footprint as your bike anyways (15-20 inches x slightly longer than the length of your bike).

Anyhow...

Good stuff:

1) Will help your spin and get you to pedal in circles.
2) Forces you to hold a really straight line.
3) Feels more like "the road" than a trainer

Not so good stuff:

1) Harder than a trainer
2) Requires more concentration than a trainer.
3) Probably won't be able to hold your line very well while wearing your beer helmet.
4) Prohibitively expensive. You pretty much have to get them used. Good news is that the cheaper Challenger model is better than the smaller more expensive ones and my roommate's dad, who's a world-class track sprinter type, swears by them.