Mine came with the factory 8-rounder and I bought 3 or 4 more Midway 9-rounders that I have been most happy with. I should buy more. This will probably become one of my New Year's Resolutions: get more mags for the 3904.
I have lots of mags for my Glock, which I do not personally care for all that much. The Glock has no "double-strike" capability, and down here, you takes what primers you gets. Often, even perfectly seated primers of crappy origin can cause the odd misfire on the first strike, especially using the Glock. I actually don't recall EVER having a misfire with the 3904. With the Glock, your goose could be cooked, never mind the 15 round magazine (or even 17, or more!).
To me, trapped in my "takes what you gets" World -- albiet with near-perfect weather and latina ladies being what you gets -- the S&W is just a much better handgun to have to stake your neck on. I wish it were the stainless model, but I'm very happy to just have it in any case.