Review writing can be a great way to build a writing portfolio and experience. It is naturally a bit of a different beast than fiction writing and blog article writing, but can be enjoyable as well. In some cases it can also be a way to generate revenue from your writing.
Personally my criteria for what to write reviews on is that it has to be something I enjoy and I am passionate about. On EpicPlains.com I review books that I have read or have been sent review. I love reading new things and sharing my opinions on what I read, so doing the reviews is great for me and great for the site.
Over on OCIA.net, I write computer hardware reviews. Now that is an extremely different style than book reviews or writing for blog posts. Yet, PC hardware is a passion for me. I am a computer geek and love it. I get sent the latest in PC hardware and get a chance to kick it around to see how it works. Then I get to share my opinion on it and have it posted to that site. I also get to work on my photography as well at the same time. I also has a great editor over there who makes sure I get the article right and that the quality of both the writing and pictures are up to par. It is good experience to bring to my other projects and my own sites.
Writing reviews is something any of us can do. It takes a bit of practice to do it well. I continually evaluate how I am writing reviews to see if there is something I can be doing better. I try to carry that same process over to my general blogging and my other writing projects. It might be something to try on your own site. If you do not have your own site and would like to try your hand at book reviews, take a look at this post.