I typically build an "About Us" type of page on Google Sites when I am using Google+, YouTube or any other Google-owned channel for promotion of a site (since it comes free with a Google user ID anyway) but have never seen significant traffic to one of my GS pages, or from one of them to any of my sites. However, they can rank well in SERPs, particularly if you're targeting your brand rather than your product, and it never hurts to dominate the front page of search results for your name with pages you control.
As far as building a fully-fledged site there, I have never before tried it -- I have found domain redirection and masking to be a hassle with Google Sites, and sites.google.com/site/etc is a crap thing to try to share as a URL. So that alone would tend to turn me off to the idea, over and above the fact that if I run things on my own space I never have to worry about keeping track of somebody else's terms of service.
Frank