I love Tabata!
I generally workout 2 times per day. My lunch session is usually a 30 minute run which is 5 minute warm up jog, 15 minutes at a 10k pace then either sprint intervals or 15% grade intervals.... as soon as I finish that I do Tabatas using calisthenic and plyometric exercises. I'll do 1-2 Tabatas to cover different muscle groups and balance pushing/pulling.
I have several MP3's I use that ring a fight bell for 20 seconds on and 10 seconds off, overlaid over top of some upbeat songs... 8 sets total, after 4 minutes you're done.
I vary my Tabatas, but typically I do 30 pushups in 20 seconds at least 3x during the session. Other exercises I mix in include kimura situps, plyometric burpees with a tuck jump, pull-ups, jumping jacks, plyometric jumping straight up as high as I can, high knees, air squats, squat kicks.
The key with Tabatas is you are to go as hard as you can for the 20 seconds on. If you push hard, they are hella crazy.
There's very good science behind them, a lot of elite athletes use Tabata.
One thing to watch with the gym sessions is I've seen them offer "Tabata" that isn't Tabata at all.
The key to Tabata is a negative work/rest ratio with maximum intensity. You dont work during the 10 seconds off, I've seen stupid gyms do things like push 20 seconds work and then 10 seconds in some isometric tension... they're idiots if they're doing crap like that and I wouldn't go.
You can do Tabatas with virtually any type of exercise you want... from running sprints, to weight circuits, to cardio circuits, etc. It's all about intensity and negative work/rest ratio.