I am an attorney and not associated with any prepaid. I think that sometimes they are not a bad deal when they are a freebie-I think some union members get them. They are okay for getting a house closing done, a simple will. I have also heard that if you get the right plan they are useful in getting you an attorney to handle a traffic ticket out of state-like for truck drivers and such. I do know a good local attorney who does work with them.
However one of the worst real estate closings I ever did was with a prepaid attorney on the other side. Instead of the $ 600 or so he would normally be paid he was getting $ 250 or somesuch and just did not seem motivated.
I particularly don't think much of them for criminal work. If you get arrested you don't want a random attorney, you want an experienced local attorney who does a lot of criminal work. You wouldn't buy a cheap carry gun, don't get a cheap criminal defense attorney. If you are in that much trouble you need a good one.
I think a big part of hiring an attorney is getting someone competent and that you like and are confident in. You are giving up that in exchange for getting the services (that you might not need) for a reduced rate. But you can always call around and try and negotiate a reasonable rate if you need an attorney. I'd say don't bother.