Aikido for three years; BJJ for six weeks.
I also do an MMA class every week.
I easily handle all other beginners, including a few very big, very strong guys, in BJJ; in my first ever class I sparred with a white belt who's trained for three or four years, and submitted him with a kimura.
I rolled with a very experienced, talented, and respected blue belt on Wednesday; he told me my top-game is on a par with someone who's been training for six months.
So does ability in aikido transfer to other martial arts/martial arts in general?
The answer is very obviously: yes.
Ah, it's hard to say. How much is your natural ability, and how much is the Aikido? When I started BJJ, I had five years of Aikido, and I got tapped out by everyone in class that day. I did make progress very fast, and by my second year was able to give most of the purple and brown belts a run for their money. Lot's of my class mates said that this must be because of my Aikido training, but again, how much is my past Aikido training and how much is my natural ability and dedication to training?
When I did Kendo, Aikido put me a bit behind in a few things. I had to drop a few habits that Aikido weapons work had engrained into me. So that was a problem that came from previous Aikido training. It's really hard to tell.
One thing that is clear, Aikido doesn't teach the kind of techniques and strategies taught in MMA.