During some initial testing, its helping my site quite a lot. I had a lot of external scripts, large images on my homepage, and just by setting up CDN. My load time went from 10 seconds to about 3-4 on average.
And so far, I think (not that I checked for it) I have been charged once $0.17 for the week my site's been up.
One thing I read everywhere is try to use your own subdomain CNAMEs and avoid using the domain name Cloudfront gives you (mostly for SEO purposes).
Other than that, its actually been pretty good, just don't expect to change your style.css after activating cloudfront. I had some issues getting it to read my editing in style.css.