darsben gave sound advice, I think. So many of the rants against a road service stem from expectations well beyond what the contract promises, which is probably the result of salesman BS and buyer ignorance.
Good Sam isn't all bad (or all good), but the others have their share of complaints as well. And they all have the same Achilles's heel, which is their dependence on independent mobile service providers everywhere. If you encounter a situation where the local service is busy or not very near to you, or the service employee is not very skilled or very ethical, or the road service is limited to towing you elsewhere, you are probably going to be unhappy.
I've had policies with GS, Coach Net, and Allstate Motor Club. Have had both good and not so good experiences, mostly dependent on the timing and location of the breakdown. I used to recommend Coach Net but since they became part of NMC I now feel their customer service & coverage is no better than any of the others. I'd shop for the best price for the level of service I wanted and change each year or so if necessary.
Despite the above, I recommend having a roadside service contract. Few things give more peace of mind than being able to call for help when your camping trip suddenly goes into the crapper with a mechanical breakdown or you get stuck in mud or a ditch you didn't see. The help may be less timely than you wish or cost you some extra, but for most of owners it's still a godsend.