In terms of a mini it does have quite a bit of space, but I know for me it would be too small as a diaper bag. I guess it really depends on how much you plan to carry. and the number and age(s) of your kid(s). Stroller where you can store a bottle of water? Is it diapers and wipes or juice boxes and snacks? Do you need to throw in the odd bit of clothing? A book? How big is your wallet? All of that stuff. I've attached a pic of me carrying my mini Pom, keep in mind I'm 5'1" and under 100 lbs. I've also attached one of my usual stuff in it, a full sized wallet, my sunnies, hand cream, inhaler, little note book. Then a third of the same stuff but with a book added (the book is 9" tall x 6" wide and is touching the bottom of the bag). For me, it's great for my day to day stuff but there's not a lot of space for extras, like a water bottle, book, etc. A couple of diapers and a pack of wipes might find your bag pretty full on top of your day to day stuff.
IMHO the best diaper bag would be a men's Day. Not the women's. The men's you can wear cross body, or, if you wear it on your shoulder, you can get into it without taking it off of your arm, a huge plus when you've got littles with you (my twins are now 22, but I know of which I speak!). it also sits flatter against your body than a mini pom would, and would be a bit easier to carrying as it molds itself to you rather than sticking out a bit like the pom does. The only thing is depending on your height you may have to have the strap shortened. The older ones adjust with a buckle, the newer ones with holes.
Buy a mini pom by all means, I love the bag. And I suspect one day I may try a regular pom, although I know they're really big, I just wouldn't use the strap but carry it on my shoulder with the handles. I love big bags. That's another thought for your diaper bag, but I think it might end up being a big black hole. The men's Day might be a bit more user friendly...see the link below for some pics of a men's day, newer version.
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http://www.ebay.com/itm/Authentic-B...353124?hash=item1a249c0024:g:4coAAOSwopRYi7dL
Unless of course, you're one of those women who somehow find bags like the mini city big enough for day to day...don't know how anyone can do that!