I think you'll probably get a lot of pro and con responses...Mirena seems to be hot topic! ;) I have it and love it. I have had no problems with it. This is the second time I've had it. The first time around it took my body about 3 months to get on a normal cycle, which means it pretty much disappeared totally with ocassional spotting. This time it took me about 4-5 months. I think those first few months of sporadic bleeding is worth it. I do have a friend who had terrible side effects and had hers taken out, but personally, that is the only "horror" story I've known.
As far as cost, it really depends on how much your insurance will pay. If you ask your doctor's office I'm sure they will call the insurance company for you and find out exactly what your part of the payment will be. I don't remember exactly, but I'm pretty sure I paid less than $200. You do have to pay for it to be removed as well, but since this is the final decision for you that won't need to happen for 5 more years. They might just roll that cost together with the placement of a new one, I don't really know...you'll have to ask your doctor.
Besides being a fan of it, I also have to say that if you know you're not having any more kids, I would hesitate to take a pill every day for the rest of your life (or until surgery is an option for either of you). I did it for a long time and I really have no problem remembering to take it, but ugh. As a mom with two kids running around, if you can avoid it, you should!