Personally I found Invisalign really inconvenient - whenever you want a drink you have to take them out so they are always coming in and out for 12 months! I found them very uncomfortable as well and tight on my teeth. Also virtually everyone I met registered them immediately so they are not "Invisible" at all.
Something else which was potentially more sinister is that after two months of treatment I developed a condition called Epiglottitis which is basically an inflammation and swelling of the epiglottis that can be fatal as it closes down your airways. George Washington died from this. I developed the condition shortly after a night out in a crowded bar when I was constantly taking the invisalign in and out in between drinks, reaching right into the back of my mouth (as instructed). I have never had any tonsil problems before and I personally believe that this caused me to pick up the bacteria which caused the infection.
You only change invisalign every two weeks and it would be easy for said bacteria to live upon the invisalign and work its way to the tonsils and then the epiglottis. This is all a theory of course but I personally believe there is a risk here which I was not warned about. I was ambulanced into hospital as an emergency and spent a week there connected to a drip, unable to eat or drink and was given a daily cocktail of adrenalin (yes that stuff Uma Thurman gets in Pulp Fiction!), steroids and serious antibiotics, I weigh 154 pounds and lost 30, nearly a fifth of my body weight. In total I was off work for two months.
Obviously I stopped the treatment, I'm 41 and in my opinion life is too short to risk for less gappy teeth!
My dentist initially told me that invisalign were very concerned about what had happened and would be in touch with me to see if there was any connection with their product, but this happened two years ago and I have heard nothing from them since.