Here me now, believe me later...
DO NOT BUY A DIAPER BAG FOR ANYONE BUT YOURSELF!
The only person to pick out a diaper bag should be the person using it. Since most women do the caregiving in this egalitarian world, let's talk about that first.
Mamas-to-be...First, do you think you're going to carry around that cute purse AND a diaper bag? You can try, but I'll bet that you'll get tired of it and start throwing your wallet, keys, and other stuff in with the rattle, wipes, and butt cream. Make sure that the diaper bag has a section FOR YOU. I had a pretty simple bag, two open sections plus a middle zippered section that held the changing pad. Come to think of it, it was pretty much like my work bag now, except in the middle is now my laptop. Hmmm....
Anywho, when out shopping for a bag, empty your purse essentials in there and then see if you have enough room for baby stuff. Yeah, put your stuff in there first. Selfish? Maybe, but you're gonna be pissed if you're out and can't find lip balm.
For daddies there are man bags out there, but I haven't tested any of them and can't tell if they are worth it versus just taking the mommy purse stuff out of the bag. Any daddy baggers reading this?
For most of us parents, the diaper bag will be our constant companion for at least 2 years, so choose wisely. Then when you find it, put it on your registry.