I am a dental hygienist, and there are basically two trains of thought about invisilign. They are appealing to adults who don't want the typical braces look and I have seen them give really nice results.
The downside is that when your an adult it's a lot harder to move teeth and bones....which is why you mostly see teens/preteens wearing braces - their mouths are still growing and developing so it's easier to maneuver. So for adults it may take a little (or a lot) longer to move and shift teeth that have been in their 'spot' for a long time because they are comfy there, and the invisilign method isn't as strong as typical braces so that will add to the time you need to wear them.
Biggest advice would be to wear them as you are supposed to, and if you are given a retainer for afterwards, wear that every night b/c your teeth will always want to move and shift back to their 'home'. Hope that wasn't too much dental talk...I am a huge nerd and could talk teeth forever LOL.