Tabata is TOUGH if you do it right. Don't plan on doing anything after it. Best exercise is the back squat. I'd suggest once a week if you're trying to lose weight, and maybe once a month if you're trying to gain.

And on the comment of taking a week off to surpass a plateau, that's some of the best advice ever. I recently took about a week off, and ate like a fatass, and am breaking PR's left and right, and gained about 3lbs of muscle in a week and a half, and 1/4 inch on my arms (and I hardly ever train arms directly).

REST DAMNIT! As hard as it may be for some people not to lift for a whole week (me included), it's worth it! Just be sure to eat big.