I have a retainer that is something like Invisalign (it's different from the traditional retainers with a thick plastic thing that lines the roof of your mouth with a few things of metal).
I had to wear them every day for 4 or 5 months or so, so I think I can give you a pretty accurate representation of what they're like.
I don't have a picture of myself wearing them but it's not like it would make that much of a difference...it's clear. You can only tell if you look at someone's teeth closely, and then you can see that it's a bit more reflective than your actual enamel.
No, it didn't turn my teeth yellow. I wonder what that's all about?
I can't answer if they work or if they hurt because I used them for a different purpose. But for me, they only hurt if I didn't wear it for a few days, but definitely not as much as when I had braces.
No, the good thing about Invisalign as a retainer is that I had a very, very minimal lisp, compared to a traditional retainer. You don't have to worry about sounding funny, although if you're still self-conscious for say, a speech or something, just take it off for a few minutes.
They're not noticeable, that's the whole point of it. They're clear and molded especially for your teeth.
They should come with a case.
Check out their site at
http://www.invisalign.com