In the last 2 months, I’ve been asked to conduct 5 reviews, all for different journals, to determine if a manuscript should be accepted or not. I performed 4 of the reviews, and declined to review one manuscript because the title and abstract alerted to me the fact that I probably wouldn’t understand one word of the paper. Before that, there was a 4 month period with no review requests, so you never know when the requests will occur.
Performing these reviews is the other side of the coin – I’ve submitted many papers of my own and read reviews of my papers. I know how it feels to have my own paper severely criticized, so I try to be gentle in my reviews when I see something wrong, but on the other hand I never have a problem in pointing out problems.
The review request contains the title and abstract; if you agree to perform the review you get the full paper. Many papers require revision which often means another review, where I can see how the authors responded to my comments.