I am 29 yrs old and am on tray 11 of 13. The results so far have been good and my teeth actually look better with the aligners in than without :) because it makes the gaps disappear and look more uniform. They also make my teeth look whiter..!
When I first got the trays, the first few days are the WORST and it's hard to get used to. I was regretting my decision at first. It is pretty painful b/c your teeth are moving and it feels like a vice grip on your jaw. But the pain subsides with a day or 2 and ibuprofen really helps. When I switch aligner trays after 2 weeks, I always do so in the evening with an ibuprofen, and I am fine. I also found the using a nail file on the buttons to make then smoother really helps. They were so sharp initially when I took the trays out, that I was getting big sores in my mouth. I smoothed them down slightly with a file and I got BIG RELIEF. It seems gross but so worth it!
Eating is a challenge when they have to removed each time. But a person gets used to it and it's great hygiene to brush 5x a day. It makes my dentist visits so easy! Speech is slightly impaired with certain syllables - like 'F' & 'V's, but it's not major. I would choose invisalign any day over traditional braces. Plus the chair time at the ortho is minimial. Every 8 weeks makes for only a handful of visits and they are super quick.
If you have ortho insurance through your dental plan, it will typically cover some of this treatment. Mine paid for half and I got a discount for paying cash with my ortho, so the cost wasn't bad at all. I would recommend finding an ORTHODONTIST versus a DENTIST for better results. Look on Invisalign website for one - especially a Premier Provider that has lots of experience.
I will need permanent retainers to keep my teeth in place afterwards but should be much easier than the trays. Overall I would recommend this for adults as an alternative to wire braces!