How good am I?

shang tsung from the time i spent with him is pretty wicked (very good). i dont use him too much cause i got too many other characters i use but yeah the guy can dish out punishment if you practice and practice
I'm going to guess that he doesn't.

I've personally never been a big fan of MK3 Shang Tsung, but I suppose he can be decent.
He's alright, and there's a lot more to learning him than just his basic moveset. His major benefits are that you can keep your opponent guessing if you can use a lot of different characters (including Shang himself), but a big drawback is that he is vulnerable when changing back to Shang. Knowing when to morph is sometimes tricky, too.

If you're trying to become a super-badass at this game, I wouldn't look to him for that end. Kabal is easily the baddest badass in the game, followed by Human Smoke (at least in UMK3). If you're wanting to get really good at this game, start with either of these guys.