Yeah, you get the wrong shit, and this stuff can cripple you. If you've got flat feet, get some custom insoles (superfeet or whatever, I don't think it matters) and get them fitted by someone who knows what's up. I like FootZone, up on Cap Hill. If you don't have flat feet, don't fix what isn't broken.
Trusting REI to treat your feet right is about as smart as trusting those boneheads' climbing gear recommendations.