I have always wanted a Petunia Pickle Bottom (since 2004 when Oprah gave one to Julia Roberts for her b/g twins). I met a lady once who was using her PPB boxy backpack with her fourth child and couldn't say enough good stuff about it.
They retail for $180 and I just found one for $150 and happily spent every dollar.
I think your style in purses can tell you quite a bit about what your diaper bag style will be. I personally pay more for purses (Coach, Dooney, etc) and then carry them for a long time vs. paying less for purses that I don't carry as long.