The Lives of Tao written by Wesley Chu is the story of Roen, an overweight middle-age man who hates his job and himself and Tao, an alien who resides within host bodies. Tao's task is to stop the Genjix, aliens like himself who intend to take over the world and rule humans. Tao's host dies one night whilst fighting the Genjix, and Tao is forced to find a new host. The only one available: Roen.
The first third of the book is quite amusing, mainly Tao reprimanding Roen for being such a huge slob and trying to get him fit and combat trained. His trainer is Sonya, a very attractive woman who has a good alien (Prophus) with her. Roen's roommate is stunned when Roen starts eating right, working out and hanging out with pretty girls. The novelty is portrayed really well.
The next third is a little boring, mainly why me, this is too hard stuff. The last third I didn't like at all. Lots of high-action fighting but not a whole lot of plot, and the timeline was hard to follow. The ending, which is supposed to be really sad, wasn't written as well as it could have been, and once again timing was a little difficult to understand.
I did enjoy the symbiotic aliens, and thought the good aliens and bad aliens teams were believable, but the rest of the book just didn't live up to the first third, progressively getting worse as you read more.