I was worried about aggression, and it's true that I have a shorter fuse since going on T, but it's controllable. Actually what freaked me out most was fronting up to the doctor and talking about the whole trans thing. I hate talking about personal stuff, and this is pretty darn personal. But I made myself do it, and it was fine. Best thing I've ever done.
As far as undesirable side effects go, be prepared for acne. Lots of acne.