Anyway, the problem involved writing a tree generation/traversal along with a little equation parser and a lot of string parsing (I think, at least). I was really uncomfortable because while it said "we will run this code" I didn't know what their expectations where (In what environment are they going to run the code? Is underscore ok? They said "you can't use built in eval" so do they really want me to write my own eval? Do they care if I look on stackoverflow for string parsing stuff?). I only had an hour for a difficult problem and I spent most of the time wondering what they really wanted and stressing out about little details that wouldn't consume ten seconds of thought on the job.
I've interviewed a lot and I'm pretty confident from a lot of experience whiteboarding code or typing up an answer on the spot in interviews, so this was very frustrating and I found it to be disrespectful of my time.