I am in the process of getting one of those for me, and my bestfriend since high school got hers put in about a year ago. She said that she had mild cramping two or three days after it was put in, and that was it. Her period stopped all together about four months ago, and took two prego tests to make sure she wasn't. Her husband says that he can't feel a thing... and that she doesn't have pain either.
She did say that you have to check to make sure it is in the proper spot every month or so. Maybe your partner is feeling it because it was "bumped" out of place. Did your doctor show you how to check it, and let you know what to feel for? My bestfriend explained to me that it is difficult to bump that thing out of place, but sometimes it happends.
Bottom line... every woman is different. If we were all the same, then there wouldn't be different alternatives to birth control. There are other ways that could be more convenient for you type of needs. Everything has it's good and bad side... it all depends on what suits you... REMEMBER THAT. Good luck!