I have had many good experiences redeeming miles on UA; as recently as last week, I booked an award trip from SNA (Orange County) to YUL (Montreal) for Labor Day weekend 2007 with ease on United. Given the limited United service from both SNA and YUL (Air Canada, Star Alliance partner, was as usual not being any help with availability), and the fact it's a major holiday weekend with a Friday departure involved, and the fact the 330 day advance window for booking these dates had already been open for about 8 full weeks, I'd say that's worthy of commendation, as the O.P. implied. ^

In contrast, I probed Delta, and got next to no options for the same dates and adjacent ones as well. One outbound option they offered had me connecting in CVG and spending the night there at my own expense.

I also agree that AA is good, based on one recent experience.
The worst, IMHO? Got to be Northwest, hands down.