It is good for contest prep when your thyroid starts shutting down, but other than that I would say to stay away. It is catabolic. Meaning, you are going to burn muscle along with bodyfat when using it, so it should be used in short duration and to supplement your thyroid production. I would keep it at 12.5 to 25 mcg a day.
There are plenty of stories where guys over-dieted using T3 and ended up looking like string beans on stage. They ended up burning 2-3 years of muscle gained on a contest prep.