Does the iphone 3gs still work?

Forgive my ignorance but i'm not used to cell phones.
My friend is trying to sell me an iphone 3gs for $50, he said there's nothing wrong with it except it has a cracked screen.
a couple of months ago his brother bought an iphone(not sure which one) but it wasn't able to be used as a cell phone because it was too old.

So my question is, if I bought this iphone could I activate it(Tmobile) and be able to use it like any normal cell phone?

DON'T BUY IT! You can get the iPhone 4 on the Apple website for FREE with a two year contract!