Yea - I'm hopping, you got a problem with that?
If I don't have something to say - then I don't. Sometimes it comes to me months later and that's fine too. I keep pretty good notes anyway so when the moment hits - I'm ready.
But back to the actual question.......
Occasionally, if I'm really loving a book and my thoughts are just flowing freely, I start writing my review while reading and finish it up when I'm done. I have never put a review up without finishing the book first though.