I have been doing it twice a week for the last 9 years. At the time I was 56 and I learned that, for me, I did much better if I limited my lifting to two days a week vice three. Toss in two days of cardio along with the Yoga, and my compulsive itch to work out gets satisfied.
I started out using instructional tapes but now have the routine memorized, so I just put on classical music and go with it. Rodney Lee puts out several good tapes. My wife used tapes for several years, but enjoys going to a regular class with an instructor. Either way, Yoga adds a somewhat different dimension to an overall fitness program. It is not a substitute for strength training or cardio; it contributes to your flexibility, sense of balance, structural alignment and it finds a lot of muscles that are not getting hit when you lift.