I am 28, and about 6 months in to my 12 month invisalign treatment. I will still have bite issues due to an improperly placed lower jaw even after this is done (which I have accepted for now to avoid jaw surgery), but in terms of straightening my teeth, invisalign has worked well.
As for the "invisible part" -- well, I think they are still fairly noticeable, and I definately talk a little strangely when they are in. And taking them out every time you want to have a snack is a real pain, to be honest. That being said, because I had braces as a kid and just needed to recorrect some of the returning issues, I have gotten away with only wearing them at night and in the morning up until lunch time. Don't tell my ortho!
I don't know about other people's experiences, but I have had way way less pain with these than with the braces as a kid (arguably I needed more correction back then) - there is virtually no pain and not even much, if any, discomfort when I put a new set in. And I only go into my ortho for about 15 minutes every 6 weeks, so its just way less time consuming than tightening and maintaining regular braces.
Because I dont have to wear them all day, which I wouldnt necessarily recommend to everyone - I think it really depends on your situation -- this has been a really good alternative to braces for me. Its only been 6 months and my teeth are almost perfectly straight. And if I have an event, like a wedding or a dinner, I just don't put them in. I would highly recommend them to the right candidate, but by all means, use a trained orthodontist instead of dentist.