Quote From metabanalysis:
Pay: no. Think of it like karma: for every paper you submit for review, you should review at least one other paper.
Advantages: (a) you get to see some of the latest research in your field. If the journal is a top journal in the field then the research is also likely to be of good quality. (b) It hones your critical thinking skills. (c) You get an insight into what journals want to see in a paper.
Disadvantages: it takes up time, especially if there are lots of changes required over two or three resubmission.
I agree with all of these, plus I would add that it looks good on your CV :)