If you're going to be using your VPN for anything bandwidth-intensive like torrenting, or keeping it on for long periods of time, a free service's drawbacks will become noticeable very quickly.
While, for example, Hotspot Shield's 500MB-a-day allowance is generous and allows you to use it every day, for activities like streaming and torrenting you'll run out pretty quickly.
Also, paid-for VPNs are more likely to have watertight privacy policies, and securing your privacy is one of the most important VPN uses. So, if you're using a VPN as a security tool you might want to think twice about where your free service is getting its revenue from.