I completely agree it depends on the child's personality/development. I've known for a while that DS is an introvert; he almost never wants to play with other kids. He's also been very attached to me, so I wasn't sure how well he'd handle being in a class without me. So we waited.
My DS is almost 5.5 and he just had his very first class today--tap dance. He enjoyed the class and is looking forward to going again next week (it runs through May). It's only 30 minutes, so not too long. We were concerned, going into it basically blind, that DS wouldn't like it and we'd be out a lot of money...but I researched their refund policy, and basically we'd only be out for a couple weeks (you pay by the month).
So you could always try to find a place that will either let you do a drop-in, just to try it out, or a place with a decent refund policy so if your DD hates it, you're not out too much money.
I chose tap b/c DS just seems to be a natural tap dancer (maybe it was all the Singin' in the Rain we watched when he was younger?). I would have put him in a tap and tumble class, but he's too old for the ones offered in our area. So it's good you're thinking about this now =).