WARNING::: VERY long post!
Okay, I'm in for helping....I made my own bag because I couldn't fathom paying $180 for a diaper bag, when I could make it myself.
First off, I LOVE a backpack, I always have, but especially now with a baby. As having on my back, It doesn't get in the way when I put her in the car seat or, most importantly, carry her on my hip. However, I designed it so that it could be a shoulder strap too, with D rings at all four corners and clips on both ends of the two straps.
Second, a wipee and diaper section, either removable or built in the front (read easy to get to) is essential, and probably goes without saying.
As for bottle holders, I find that most prefab bags go overboard with them. Personally, I only need one inside.
Also, my diaper bag IS my purse, as is the case for MANY of my mommy friends. What I wish I had done was design a removable section that is pretty on its own, that I keep my essentials in, ie, wallet, keys, lippy, and such, that when I go out without baby, I could just pop that out and be on my way. Switching purses is a hassle and usually, I'm rushed.
As for other essentials, here's what's in my bag right now. Diapers, wipes, butt cream, wallet, coupon keeper (gotta be thrifty), camera (I ALWAYS have this at the ready), my small cosmetic bag, snack cup, empty bottle, little bags of powered formula already measured out all in a ziplock bag, phone, bluetooth, and a ziplock bag with a baby pharmacy in it (asprin, teething tablets, sunblock stick, motrin, gripe water when she was littler).
Granted, I carry alot of crap around with me, probably too much really. But, I always have a band aid!!
Oh, and I wanted to link to the bag that I modeled mine after. The thing I LOVED about this bag, and copied, is the fold out changing pad. And let me tell you, I use the heck out of it.....especially on public changing stations. I made it removable as well, so I can throw it in the wash when it's too germy....
Here is the link: http://www.petuniapicklebottom.com/collections/original/boxybackpacks/
Sorry for the long post, it's just that I am always on the quest for the PERFECT bag and so I feel the tug when someone asks my opinion!