I started using Invisalign in 2005. Teeth weren't terrible, but the gap between my top two had always bothered me. I had to wear the top retainer for 5 months and the bottom for 9 months. Other than the hassle of having to remove them to eat and having my speech affected slightly, it was definitely something I would do over again.
I wore the retainers every night for 1 year after I completed my treatment and was able to retain the result. Then I got lazy and didn't wear them very much for 5 or 6 months since my dentist said I had to use the retainers for a year only. I do not recommend this at all. Getting them back in was painful since they had moved a lot in that time. Now I am back to every night, but my teeth haven't looked as good since.
If you spend the time and money to do Invisalign, know that you will be wearing the retainers for a long, long time after treatment. The bright side is that you can whiten your teeth freely with them and they provide the added bonus of protecting your teeth from the effects of grinding.