Everyone has a different opinion on this; in my view, very little. Sure, everyone knows it's the terminal degree in the field of creative writing, which qualifies someone to teach creative writing at the college/university level. But a lot of creative writing profs don't have an MFA; I even know a couple of directors of creative writing programs who don't, and they are very well respected poets. In the end, the degree alone isn't likely to get you a position teaching creative writing--considering the many many people who possess it these days--what matters much more, I've learned from many sources, is a strong publication record.
Other people will tell you it'll help you make "connections." But really, what does that mean? That editors are more likely to choose your material for their publication if you're connected? I'd be skeptical about that. That you'll likely meet someone in the program who knows someone who knows someone who can offer you a job? As vital as networking is when you're looking for a position in any industry, it still boils down to who the best candidate is. In short, I really don't think an MFA holds any professional promises whatsoever. It is not guaranteed to make you a creative writing professor, a published writer, the editor of a magazine, etc. etc. All it promises is that you'll have 2-3 years to "hone" your writing and to read, read, read a lot of good literature. Which is even better done if it's done outside of a degree program, if you ask me.
Anyone else want to comment?