Re: A question specificallly for those experienced in Aikido and Judo
I don't see aikido being as useful to judo as judo is to aikido.
Seriously, and I'll tell ya why.
Judo is a sport with a very limited set of rules. Most aikido techniques fall outside of that ruleset. I'd venture to say a lot of the theories and postures aikidoka use are actually bad for judo randori. While there are some useful parts of aikido for judo, the majority of aikido simple does not translate well to judo.
Aikido however is lacking (at least the aikido I've seen) in the direct confrontation skills of judo. The randori of judo, the body contact throws, footsweeps, and other techniques of judo can be used in an aiki sense. The two biggest things judo gives aikido is resistance based drills/randori and ground work. These are two areas that I feel are majorly important that are often glossed over by aikidoka.
In terms of a whole self defense package, I think combining the two is better then either one alone. Aikido is going to make you think more about striking and blending (vs the push pull of judo) and judo is going to teach you to deal with adversity and stay relaxed under intense pressure. Judo will also help if the fight goes to the ground, and will help you with physical conditioning and hard body contact throws and foot sweeps often neglected in aikido.
Also people who study judo or any standing grappling sport seem to have a much better sense of when a person is off balance then those who dont.