Here's some answers based on my sister's experience with it...
She had a front tooth that was starting to almost overlap the other front tooth, so her teeth were considered pretty crooked to start with. Invisalign worked for her in a little under a year. She said it really hurt each time she had to have new trays made (because they feel a little tighter each time, as they're shifting your teeth). Her speech was only affected by it the first couple days, then it was fine.
It was noticeable when you were in *close* range to her, like a couple feet away or closer. From far away it was not noticeable and it wasn't noticeable in photos. I have a coworker who I just noticed had Invisalign. She's had it for months and I talk to her everyday. So I think it's probably only noticeable if you know it's there and you're looking for it, kwim?
It was a little disgusting to watch her take them out and put them in when eating, but she got better at it as time went on... there was a lot of snapping and crunching noises, and sometimes lots of drool LOL. Also, you cannot have anything besides water when you have them in, or you risk getting cavities and staining your teeth... that is really important!
Her teeth are perfectly straight now, but she still has to wear them at night, probably forever. She feels like it was definitely worth it.