I bought a pair of orthotics from www.ourfootdoctor.com and have had a great time with them. I only started dancing last October (2005) and didn't get the orthotics until around May. I feel that my dancing improved greatly because of the added support I get from them. But, just so you know, I have moderate to severe arthritis in my feet so support is something I needed. My balance improved, my ability to do a decent rise and fall improved and my duration improved.
My only complaint with these orthotics, and I'm not sure if I would find the same thing with others, is the tendancy for the orthotic to slip forward in my shoe as I'm wearing. This could be solved right out of the box by raising the back edge of the orthotic about half an inch. Right now, they're just not high enough. I solved the problem with a little dab of rubber cement in the heel of the shoe. But, it would be nice if I didn't have to do that.