I have been doing a lot of code review lately, and I am unsure of the positive and negative effects and professionalism of putting positive and/or funny comments in code reviews.

We use Github as our code review platform on my team, so the comments are viewable by anyone.  I generally try to use this platform so the entire process from start to finish is visible and historical.

It's important to highlight positives as well as negatives. I know if I were reviewing the refactor of a particular hellish subsystem into something neat and clean, I'd probably buy the programmer a pizza for his efforts.

If you're using reviews as training, it's doubly important - highlighting a good piece of code will be helpful for the junior programmers also reviewing that code. They will have a chance to ask questions about why a particular approach or technique is better than another.
