Disclaimer - I am not a dancer, but I just wanted to try these.
Wow! These are SO comfy. I am always very picky about my underwear (no exposed waistband, scratchy seams etc.), but these are very comfortable. The material is very soft, and I haven't noticed the seams yet, as they seem to be stitched pretty flat. I usually wear Hanes boxer-briefs or briefs, but these are a lot softer! The thong design is new to me, but I like the feel and I would definitely buy these again! I can think of only two possible cons.
1. The waistband is a little bit big, but the only reference I have to compare against is regular underwear. 2. The thong design may feel weird to some people, but if you're used to it, then great!
I can't say much in terms of support, because again, I'm not a dancer, but I have a 33-ish waist and I got a size large dance belt. I probably could maybe have gone smaller, so I said it runs a little bit large. I AM a competitive swimmer however, so I have a little bit of reference from that. I will say though, a speedo brief holds the privates in a little better than this, but this could be from the size I got. I will most definitely be ordering again, and who knows... I might get into ballet from this!