Ahh, this thread again.
First - Do a search on Sherdog discussing the effectiveness of BJJ in street applications. You'll find post after post of people saying "I don't train for self defense" and "The best form of self defense is to run away" etc. Interestingly enough, when threads like these come up, the response is, "Aikido is crap. That shit would never work on the street."
So, they don't practice BJJ for self defense, and in a situation where they might be attacked they're going to do everything they can to not engage... but Aikido sucks because it doesn't work in the street. Confused? Me too.
People really only practice martial arts for a couple of different reasons.
1) It's fun.
B) For self defense.
4) For fitness
I'll assume that people who are suggesting that they don't train BJJ for self defense must be practicing it because they enjoy it, because they want to get into shape, or some combination of the two. Yet if someone were to suggest they might want to train Aikido for the same reasons... well... you see the response.
Is Aikido effective when it comes to hand-to-hand combat? Well, I think as we know, the individual has a huge impact on that answer. How the training is done, how serious they are, etc. In general, however? I'm going to say that it's not as effective as a number of other martial arts out there, including Judo, BJJ, boxing, etc... but more effective than not ever doing any type of training.
In my opinion, if there is one thing that Aikido has to offer individuals who participate combats sports, it is their footwork. Aikidoka have amazing footwork and tai sabaki. If my footwork were as excellent as some Aikidoka I have seen, I am quite positive that my Judo, BJJ and boxing would ALL benefit greatly.