R., as for the 1st part of your question, I wouldn't recommend for an 11 year old necessarily. It seems to me more like something a grown up woman that is researching coping with her monthly would look into and use. That's just me, of course.
As for experience, I have used this for about 2 years pre-pregnancy and loved it. I did not have any problems with discomfort or leaks although the key for me was relaxing and not being in a hurry when I was "installing" it. What was really great about it is that I could wear it ALL day and not have to dump it until I got home from work which was awesome, also great when on road trips, not to worry about accidents or messing with pads/tampons, etc.
Post pregnancy, I have had a ridiculous growth of my endometrial lining and my flow is so heavy that I have to empty it out every 4 hours which I am NOT willing to mess with outside of my home. Too messy and I am a germaphobe.
Someone else posted the link to DivaCup's website, I highly recommend checking out their FAQ's, that's really was helped me decide whether I wanted to try it or not and answered a lot of wierd questions that popped into my brain during and after shopping for this thing.
Good luck!