Pros
Started my career with a Big 4, but left for BDO as I preferred BDO clientelle (in my region) over the Big 4's; I find small and mid-cap more challenging then the typical, bureaucratic large cap. The salary bump to jump ship, laterally, didn't hurt either. Also, I didn't want to get trapped and work on only 1-3 clients, in the same industry, year-round, as I was doing with the Big 4. Since I never intended to be a "public lifer," I wanted to make myself as "recruitable," or have as many industry options as possible when I left, so the "generalist" specialty worked better for me.
Cons
Although I spent much more time and gained much more "resume big ticket" skills with BDO than I did @ a Big 4, I still get more "street cred" for my Big 4 heritage; the Big 4 stint is all people want to talk about. Collectively, the Big 4 push out much more bodies than the Nationals/Regionals, so they're probably the ones doing the hiring, and sadly, some can still be a little snobby when it comes to assuming that someone from a National/Regional is less equiped than someone from Big 4, and dare I say it...forget about it if you come from a local firm. Understandably, it's a reputation and perception thing, and while I don't agree with it, it's just that way it is. So while I feel my current success is more attributable to BDO, I would advise people to really think about the perception that comes with a National/Regional and how that plays into your future plans; you may want to consider going to a Big 4 just for the name eventhough your experiences might not be as diverse, or even better, doing both Big 4 and National/Regional. It's kind of like, the Big 4 name gets me the interviews, but my BDO experiences enable me to back it up when I get there.