I've used undercover tourist, ticketmania, and Disney will-call.
I found UCT's service to be slightly more professional than TM's---I had a small issue with TM that was more difficult to resolve than it needed to be. I also like UCT's refund policy. Their tickets come to you sealed in plastic. If you don't break the seal, you can return them for a refund less 5%.
All that said, I'm just as likely to order through Disney Will Call. The discounts offered are *very* modest---rarely more than 5% or so---and the extra cost is offset by the convenience of (a) being able to pick them up with no hassle when you arrive, (b) having the biometrics grouped, and (c) the receipt is sufficient to replace the tickets should you lose them; no need to make a photocopy.