I've to an OiOi hobo and haven't yet had a chance to use it properly (it's packed up as my labor bag at the moment). I bought it because like you I wanted to be able to use my nappy bag as a handbag and also have something that was semi-stylish. I ended up spending a bit more than originally planned and going for a leather one, cos I figure I will use the bag for other things when its life as a nappy bag is over.

I wouldn't worry about only having the one bottle warmer. If you need an extra one or two you can always buy them seperately.