I LOVE Mirena. Same opinion as the rest - extremely uncomfortable at insertion. I have already had 4 kids and many D&Cs and this was right up there with the discomfort, but I would do it again.
No weight gain.
But one weird thing...The first year I had it, my boyfriend's penis could feel it during deep penetration. Sorry if TMI. Good news, 4 years later, it can no longer be felt by that particular appendage.

Seems like the Mirena strand has retracted up somewhere in my cervix.
OMG. Did I really just post that? Gotta love anonymous forums! haha