If you plan to ride offroad where there will be much bouncing, buy panniers that will not detach from the rack (Arkel, Ortlieb, maybe a few others). If you are touring on roads, many other less expensive options are available.
For the money I think Axiom Typhoons are the best bet for road touring. Here's one site that compared Ortlieb and Axiom.
I use Nashbar's waterproof panniers. The more common problem with Nashbar's pannier, as reported on crazguyonabike.com is that the hook rivets pull through the backing. I noticed this was a possibility with mine, and I solved that by removing the rivets, adding a little square lexan backing inside the pannier where the hooks mount, and used washers/bolts to hold the hook in place. Now these panniers are tough and work well (I've had them for 5+ years of communting and touring).
However, if I bought new panniers today, I'd try the Axioms Typhoons since I do only road touring, otherwise I would buy the Arkel or Ortlieb panniers.
(If you look around you can find new Axiom Typhoons for about $65 [ebay] to $75 [aebike.com], but many try to sell them for $100+.)