I had traditional braces in my 20's, but was unhappy with the results so I tried Invisalign in my 40's. I was told the procedure would take 6 months followed by a retainer to be worn at night. The procedure was so problematic that it took 2 years and I am still using the retainer in my 50's. My problems could not be corrected successfully, so now I cannot close by mouth correctly. During the daytime when I am not wearing the retainer, my teeth are slowly forced into a crooked position which allows me to close my bite after about 5 hours. Every evening the retainer straightens up the teeth again and the following day, I go through hours of being unable to close my mouth properly until the pressure of trying to close my bite forces the teeth into a crooked position again.
The doctor was very nice and compassionate, but I now have jaw pain like I never had before and my teeth will never retain a straight configuration because if they did, my bite would not close. Basically, I am straightening my teeth over and over again on a daily basis, so I guess you could say that I am not really using the retainer for its intended purpose.
Save yourself the aggravation and go through traditional braces or nothing at all. Invisalign does not work!

11/12/08
I absolutely have this exact same problem. My teeth straightened within 6 months (shorter than the projected time), but now I'm in maintenance and it is terrible! I am straightening my teeth every night with the retainer over and over again. I take it out in the morning and they are crooked again in 4-5 hours and continue to worsen throughout the day. Also, wearing the invisalign only on the top, where I needed it, has caused my previously FLAWLESS bottom teeth to get all crooked and messed up. So now I have two sets of crooked teeth instead of one. I would not recommend invisalign for this reason only.
12/11/08
These sound like severe problems. What do your orthodontists say about them? I'd like to hear a reply on this forum from a professional. This sounds very disturbing.