I enjoy books that use instant messaging and emails to tell the story (ever since reading Meg Cabot's Boy series that uses this method, I've looked for books that include things like this. Even if it is just ever so often sprinkled throughout the prose, I enjoy it). So, I really enjoyed the format of TTYL, and the chatspeak didn't bother me either. It was all very understated and not an in-your-face, over-the-top type of chatspeak.
While there wasn't a whole lot of plot, I still enjoyed it. I like that a lot of books anymore are going with the barely there plot. It works for me.
I also liked the characters of Angela and Zoe. IMing doesn't provide a very good avenue for character development, but I think the characters were well written and developed as far as could be through IMing.
If you hadn't noticed, there is one third of the trio missing from my well liked characters list. That is because I feel nothing but disgust for the character of Maddie. I disliked her from beginning to end. There was nothing about her to like. She was a selfish beeyotch to be frank. I felt like skipping her IM conversations but knew that then I'd miss out on the other two characters so I didn't skip. I don't know if this was intentional for the author to make her the hated character in the book but she was.
Unfortunately, Maddie's character was so cringeworthy that I can't imagine reading the sequels if she is in them. It would be entirely masochistic so I think I'll have to pass.