Resistance bands?

[jnibean]

Does anyone use/have experience with resistance bands?  I've been doing P90X since this summer, and am currently using a chair to help me do pull ups.  But, I'm not getting any better at doing pull ups (I still can't do one unassisted...), so I know I am not doing something right.  I see in the videos that there is always someone using bands, but I have NO idea where to start.  Any helpful tips/best place to look for them/best brand etc?  Thanks!