Secondly, a VPN prevents other users downloading and/or uploading the same files as you from seeing your IP address. BitTorrent is a P2P, or peer-to-peer, protocol. That means everyone who uses the same torrent file is connected in what’s known as a “swarm”. Each device connected to the swarm can see all of the other IP addresses of all the other devices in the swarm. Many BitTorrent clients even allow you to view a list of other devices you’re connected to when leeching or seeding files on the network.
If you are going to use BitTorrent for whatever reason, good luck to you. If you are going to use a VPN, more power to you. But be sure that you take the time to read the VPN's terms of service before you start. And be aware of the local laws and possible penalties before you start, whatever your willingness to obey them. "I didn't know the law," or "I don't agree with the law," won't hold up as defenses in a court, so make sure you can live with any potential punishments should you choose to do something legally dubious.
If you frequent ThePirateBay, uTorrent, RARBG, Putlocker or KickassTorrents, however, chances are what you download from these torrenting sites is not legal. Government authorities can fine you for committing a civil offense, while ISPs and copyright holders will threaten and in some cases follow through on legal action. While it’s unlikely that a record company will take someone to court, they might seek damages through settlements.
You’ll get slower download speeds. Running your connection through another server inevitably slows you down, though how much depends on what torrent you’re downloading, who from, and a lot of other factors. In my experience, more popular torrents stayed at their top speed of 3.4 MB/s (my bandwidth cap) with a proxy, while other less popular torrents slowed down from 1 MB/s to about 500-600 kB/s. Your mileage may vary. I lost significantly less speed with Private Internet Access than I did with BTGuard, though.