How Effective Are VPN's?

I've been using PIA (Private Internet Access) for a year now, but without any positive feedback I have no idea how effective it really is (though I guess not having any issues is a testament in itself).

in general, how effective are VPN's?

And what is their 'breaking point'(so to speak)? I know how VPN's work, I'm just wondering how effective they are at different tasks (like torrenting, general browsing, hacking etc).