As has been stated before, the instructor is extremely important factor.
However, IMHO Judo can be one of the more effective of the martial arts from the standpoint that you regularly practice against a stongly resisting opponent.
Judo, Wrestling, Brazilian Jui-Jitsu, Boxing, Muy-Tai and perhaps a few are examples of arts where your opponent is actually trying to either pin you, force you to submit, or knock you out. These are MUCH closer simulations for self defense than point sparring or saying "I could knee you in the groin if you did that" (whiney voice).
In Judo you are tested almost every time you practice to see if you TRUELY know how to make that throw, armlock, block, etc. work against someone who is trying to do the same to you. To me it is an excellent balance between practicing safely and developing EFFECTIVE self defense techniques.
Martin
PS The following is a good forum for martial arts related stuff:
http://www.fightingarts.com/