My husband I registered for it last year. We had done the marathon and really enjoyed training together, so we thought the triathlon would be a fun thing to try (plus an excuse for another trip to WDW!)
Unfortunately, we didn't feel like we trained enough for the swim and after seeing how much money it would cost us to fly our bikes down with us, we decided not to do it after all. I still kind of wish that we did it, though, and maybe we'll try again someday.
We did go on the trip anyway, and I have to say I was really surprised by how low-key the triathlon was. If we hadn't registered, we would have never even known it was going on. The only sign of it we saw was a single traffic sign out by the Contemporary. Not at all like the marathon, where you can't miss the fact that a big event is happening.
I didn't have much luck finding information about it either when we were training, so if you go ahead with it, I hope you'll come back and post a report. Good luck!