Dear R.,
I am going to give you as balanced an answer as I can. I had Mirena for just over 3 years. I did gain about 10 pounds, but in all honesty the last year I finally got my diet in control and ended up losing 15. I can't completely blame Mirena.
I had some cramping and heavy flow when first inserted, but it subsided within 3-4 days. I then didn't have any period for next 3 years. Honestly, that was a perk. It is easy, no-thought birth control.
On the other hand, I think I suffered from moderate to severe moodiness and some depression. I felt in a fog a lot of the time and experienced a lot of negative thoughts. Mind you, at the time, I had no idea it was MIrena as the first 6 months seemed to affect me very little. By the year mark I started experiencing more moodiness in the 2nd and 3rd year it was extreme. Depression ebbed and flowed, but was generally present at some level.
Until I started doing some research considering removal in order to have another baby, I had no idea there were so many negative side effects. My doctor did not inform me of anything negative! It seems like a lot of doctors ignore testimonials and say the patients must have other problems. In truth, I myself wasn't certain it was Mirena causing any of my issues UNTIL I got it removed in January 2010. I was amazed at the difference within a few days! After the removal, though, I notice my periods are accompanied with severe PMS - something I never had prior to my pregnancy or Mirena. There are a few days now where I feel like I back in the years with MIrena. Interestingly though, even that PMS has lessened in the last 8 months of having it out. I feel like the hormones are somehow still in my system.
Keep researching (I wish I had) prior to insertion. If you get it, keep a log of any changes in mood, weight, etc. so you know if it is affecting you negatively.
Good luck!