I've also got a really weak lower back, and I was told to do hyperextensions every other day to help strengthen. I started out doing 3 sets of 5, and I'm up to 3 sets of 15 after 2 months. However, I had someone there correcting my form the first time I did 'em so I didn't make things worse.
Crunches still kill me after the first set of 25, and I occasionally have to stop during my second set, or it gets hard to breathe. I personally like to do crunches with my back resting on an aerobics ball. I can usally get more out before I have to stop, if I have to stop at all.
The roman chair doesn't hurt my back, but I'm still not strong enough to bring my legs all the way up.