VPN Services is simply a way to mask what you are doing online, sometimes its to get through ISP firewalls, or you want to cover your tracks. Sometimes you want to encrypt your BitTorrent traffic so the ISP doesn’t block it (KEMS), other times you want to get around government firewalls and use different services and visit certain sites. I for one use VPNs all the time so I can access US only services and register as if I am there, like Hulu Streaming and other media services.
I split them into two recommendations, ones specific to Torrent VPNs and the other just VPN service, but the Torrent VPNs are also good for VPN usage. VPNs do have an effect, they do slow down the connection just slightly but with some providers it slows down only 5% and with others it slows down 25%, for fast VPN connections I would go with StrongVPN but for BitTorrent specific VPNs I would go with BTGuard.
For BT VPNS:
Check out the link to TorrentFreak to give you the break down of the VPN Services that really protect you.