Juha over at Universal Rule also has two excellent blog entries on books and writing.
Having immediate feedback on your writing (and your thoughts) encourages you to keep on writing. The feedback also helps in the creative process. In micropublishing like weblogs you don’t have to publish a polished text, you can start with half-ready ideas and get feedback where to go from there. The weblog community is not individual writers fighting for the place on the printed page.
During the last four years I have reviewed about 60 books in the Tietoyhteys magazine, where I have been editing the book review column. Most of the reviews have been written by me, although I have been fortunate to find a couple of colleagues interested in reviewing books. Writing a book review is never a straightforward task, but it can be very rewarding. Writing a review forces you to ask: Did I understand what the writer intended to say? Often you find that you didn’t.