I'm part of the backpack club as well, abandoning both panniers and messenger bags in the past. The secret to finding a good pack is to:
- get one with a well-padded back
- waist straps are better than hip straps for stabilizing a small load
- don't get the biggest pack you can find; pack smart/light
- try a bunch of packs on before making your decision, loading them up with whatever you might take with you on your commute
- when you're trying on the packs, assume a biking position [bend over ]
- alpine climbing packs can make good bike packs because they tend to be narrow and often are cut so they don't interfere with a helmet
- don't cheap out! You're saving $12/day on gas, put some of that money into a decent pack