Reviewing and evaluating is part of our job; if you think about it, it's most of our job as professors. We're paid for it, in one way or another: salaries (for teaching); books or checks, for manuscript reviews for presses; a "professional service" line on the cv for being a manuscript reader for a journal.
If I'm honest about it, though, the real reward for some of this is just plain praise. I was really pleased when a number of the students this summer took the time to say (in turning in their last project) "thanks--your comments really helped" or "I learned a lot" or "I really enjoyed the class." Maybe I'm naive; a cynic might say that they're trying to ingratiate themselves so that a softened-up, benevolent Dr. Undine will go easy on the grading. Since all the grading is pretty straightforward, however, and (in an online class) there's no wiggle room for "participation," I'd like to think that they were sincere.
The same holds true for reviewing. Although there's a pro forma quality to thanking the reviewers in the acknowledgments part of a book, when an editor this week took the time to thank me for my comments and pointed out the ways in which they'd be helpful for the author, it made my day. Eventually I'll fill out the forms and collect my check (or books), but right now, I'm still basking in being paid in praise.