I'm 3 months into my 11 month stint and it is working amazingly. I can already see a huge difference and my top teeth especially are much straighter. The first set of trays are obviously going to feel weird, and yes, everyone has trouble getting them off! You feel like you're going to pull your teeth out but as soon as you've had a couple of days' practice, they pop off easily.
The only discomfort comes on day one or two of the new trays and I have found that the most recent new set has caused particular discomfort - but this could be because there is one particularly tricky tooth to fix and this set of trays seems to be focusing on this tooth. However, I've only had this set on for 3 days, so am hoping it will be fine soon. I have been having the odd Ibuprofen to help.
I've already seen what my teeth will look like at the end of the treatment and I cannot wait. After years of smiling with my mouth closed in photos, I can't wait to smile properly. I've been so pleased with treatment etc so far that my boyfriend, best friend and a work colleague have all got Invisalign too.
I've also noted some people have not found it invisible and are really disappointed. I have to say that this is not the case at all for me - and remember, you are probably going to notice it more than most as it's your face and you know every part of it intimately, whereas other people don't and they won't be searching for signs of a brace in the same way you are. I had mine on for weeks and until I told friends, no-one had noticed. Obviously when i told them they were like, "oh yeah, I can see it, but only because you told me it's there." I also didn't experience any lisping etc that some people mention. Only side affect is that the material of the brace makes my mouth quite dry - although drinking more water can only be a good thing I guess!
All in all, a great experience and I'd say go for it. Part of the package I get also gives you a full bleach at the end too, so straight, super white teeth - I can hardly wait!