depends a lot on what you're going to do with them.

For me, I just need something to haul a bunch of crud when I'm commuting. Nashbar ATB panniers work for me, and the go on sale for about $25 for two, and they're big. I've got a couple thousand miles on them and no failures yet, or even worn spots.

If your needs are similar, check them out. If you're going on a month-long tour, spend more money, probably a lot more, for stuff with internal dividers, waterproofing, better hardware, etc. I saw some OK looking ones at REI, but I've never used them. Something else to look at.