D.C.
I had spotting for about 4 months after - which was annoying because, like you, with my previous child I didn't get a period back at all until he weaned.
However, other than that, I had (and have) no side effects at all. And I lost all my baby weight within 6 months, so it definitely did not make me gain weight. The only side effect I have is a good one - when my child did wean and my periods came back, they were very light and short - spotting for a day or two and that's it. Now (3 years later) they have stopped altogether. Mirena is really great for me.