Although I've only played around with potential exchanges, I really like the Points.com concept. Once their exchange network of partners is built out (it appears that they are really in a soft launch phase now), they promise a 72 hours maximum transaction turn around which is important for those of us who never plan more than a week ahead. Also, even at a relatively poor exchange rate, I'd be happy to clear out some 10K mile balances in have in 3 or 4 of my minor programs.

If you can plan ahead, the HHonors RewardExchange offers better exchange rates, is free, but is very slow in processing the transaction (about a month).