Studied it a little bit in japan. When you say most useful, do you mean for fighting, for defense, for fitness, what?
My experience was that it's less hard on the body than karate, anyhow. There were a bunch of half crippled old karate black belts in the aikido classes who were training there because Aikido is less hard on the body. Those old karate guys seemed to be all mauled by the karate "toughening" exercises. They were damned tough, but they were also all screwed up like old hockey players.
Interestingly, most of the aikido teachers in the places I went in Japan were Israelis.
Aikido seemed okay, but personally I thought Judo and Taikwondo seemed more "useful" from the point of view of defense/fighting. I'm less interested in the passivity of aikido, though I respect the skill involved.
I'm actually looking for an even softer (on the body I mean) form right now -- I'm thinking tai chi kung fu, if I can find a good place to take it.
Wyst