My file is scattered all over his office! I've read some notes he sent to my insurance company and they were very detailed. Sometimes he takes notes while I'm talking and he will fill up 4 pages with stuff.
I think it depends on the therapist and what type of therapy they are doing. CBT is probably pretty straight forward. Depth therapy might have more interesting stuff in it.

I used to work with autistic adolescence and their books full of notes were very interesting.

I brought my journal in for the first time last session, I put it on the table and he was eying it up like he wanted to get his greedy little paws on it!