However, I didn't go to a class or see an instructor; I followed a DVD at home which was made by a very respected instructor who does teach class in Boston, apparently. So, even though it's only a DVD, I trust the authenticity of the moves and postures taught in it, and the guidance she gives throughout.
The upside is I don't have the stress factor of having to go out to a class, be around other people, get home probably in the dark if it's an evening deal.
The downside is that I don't get the one-on-one, instant fixing of whatever I might be doing wrong, from a live instructor right there helping me. Many people say it isn't real yoga (or pilates, or etc) if you're not doing it under the instruction of a live person. I totally get where that viewpoint is coming from, but none the less, I did really feel that I was reaping rewards from the DVD instruction. Plus, it's just "my style" to do things alone and at home if I possibly can. I've never been a "joiner" and I hate group activities with strangers.
Perhaps you could find a good DVD by a qualified and actively practicing teacher who does hold classes in "real life" and thus is giving you the same class, just on your TV? I really loved mine. It's called "Power Yoga For Every Body" and is by Barbara Benagh. She has a really soothing voice too. Don't be put off by the "power" in the title, which sounds rather over-strong -- that's only the 3rd level. The second level is very relaxing and gentle, yet effective.