In my personal experience with Marc Jacobs, they're high enough quality that they last a LONG time, and look new for months, sometimes even years, if you take good care of them. Now I haven't used the diaper bags as I am a FTM expecting in Feb, but I would be too afraid of throw-up/spit-up/other stuff ruining the bag, so I bought a cute, but inexpensive washable bag from Target for now.
Perhaps when my LO gets a bit older, I'll invest in a nice bag, but until then, just cannot fathom throw up on such a lovely, expensive bag!