If you want to shoot film, now is not a bad time to pick up a used MF camera system. Cameras without a digital upgrade path are especially reasonable. I just bought a Mamiya M 645 1000S with an 80mm f/2.8, metered prism, and 2 120 film inserts for $220 because I couldn't pass it up. It was exceptionally clean. The same equipment went for almost $3k at the campus bookstore when I was in college.
You can regularly find Hassy 501CMs with the 80mm lens for ~$1400 (about 40 cents on the dollar) or a good, clean twin lens Rolliflex for under $400 if you don't mind being limited to just the one (normal) lens. A square format camera is a great learning tool because it really teaches you to compose an image and think about what you're doing.
However, if you want to go digital you really need to make the MF back pay for itself (or be wealthy) because they run north of $12,000. The only MF cameras you can buy new that accept digital backs are the Mamiya 645 AFD II, Mamiya RZ67 II, and Hasselblad H1/H2.
But even those cameras are optic-limited in relation to the latest crop of digital backs. That is, the optics won't let you take advantage of the difference between, say, a 22MP back and a 39MP back. And while there is a quality difference between a 16MP Canon 1DS Mk II and a 22MP back on a Mamiya or Hasselblad, it isn't enough of a difference for most to justify tripling the price.
I use medium and large format cameras extensively. I'll probably purchase a reduced-size view camera, digital lenses, and digital back later this year. But I haven't had a professional need for a 35mm digital SLR, and with the technology advancing so much every 18 months it made little sense for me to buy into a system. But I believe that things are finally slowing down. If I were going to purchase a single camera system to do portrait work, I'd go with a higher-end Nikon or Canon DSLR. Unless you're going to make really huge prints (bigger than 16x20), you won't see an obvious difference between them and a digital MF system.
People who do stupid things with dangerous substances often die! -me