Tea bo is terrible.
the concept is good for morale, and to make exersise more fun.
However...
1, if you want to learn karate - learn karate
2, you'll concentrate too much on punching fast and offen that you'll forget about form. If you wanna punch for fitness, you gotta learn to punch properly first. Tae bo could damage your joints if you dont know basic martial arts 1st. By basic I'm taking 2-5 years training.
3, there are much better ways of getting fit. Interval training has proven to be the fastest way to loose weight and improve fitness. Sure tae bo will get you fit, but it'll take longer.