The cost vs. benefit of free portfolio sites can't be beat. At best, you find an audience, then draw them to your site to seal the deal. At worst, you're out the 20 minutes or so it took to upload your work. I'm on pretty much everything I could find, Flickr, Behance, Illustrationmundo, Coroflot, Dribbble...
Correct me if I'm wrong, but isn't Cargo Collective seems more like a hosted service that you'd build your website/portfolio on rather than a free portfolio site? Check out Clint Ford's site
http://cargocollective.com/ford
Paid portfolio sites should only be considered after you've exhausted every free avenue, and done some research to compare different sites' traffic, search/discovery tools, promotion efforts and fees. I've never paid for this type of thing, so I can't tell you if they're worthwhile.