Had one for ds (a fabric Caboodle bag), and barely used it for dd. A large nice regular bag, with pockets to take purse keys etc , and into which you can put all the bits and pieces like a fold up mat, wipes , spare nappies, is much more convenient (had a black Samsonite one). You'll find that you can't easily manage to carry a bulky changing bag, handbag, baby in car seat....
Had a serviceable,'proper' bag for dd1 from Avent with lots of pockets, warm pocket etc. Rucksack shaped. Hated it cos it didn't hold enough, slipped off shoulders, couldn't hang off pushchair etc. And it was UGLY (am handbag kinda gal.)
With dd2 had bee-yoo-tiful Mommybag (google them) in chinese silky fabric. Lovely and big, and looked nice. No flash pockets but didn't miss them.
Now she is 16mo, have graduated to a £2 swimbag from woolies containing a spare nappy or 2, mini pack of wipes and a pencil case containing few crayons and a mini colouring pad for each dd - and got my real handbags back. Bliss!!
I bought one from Mothercare when I had dd 2 and a bit years ago. Not sure if they still do it but they may have a similar one.
It was very good, plain black record bag type. Had 2 main zip sections in and a front zip pocket. It came with a changing mat,insulated bottle bag, dummy purse and a nappy bag.
Could easily fit in a days supply of bottles in and a never ending supply of nappies and wipes!
If you've got the money and especially if you're going to go back to work you could splash out on a designer bag that looks like a handbag but actually works like a changing bag (see storksak, hennabecca etc on the web)