Torrents are like Driving a car or owning a Gun (where legal).
It's not Using them per se - it's what you DO with them.
Shooting people or driving dangerously would be illegal (at least in most countries - although if you came here you really wonder how we don't become the "accident capital of the planet". Woe betide you if you get in the way of a large 4 x 4 hurtling down a tiny dirt track at over 110 KPH.
Downloading any copyright material without consent of the owner is ILLEGAL. (Still on this board a load of us - probably the majority have D/L various builds of Windows 7 -- but as far as P2P is concerned this is a relatively minor infraction -- people are usually more concerned with Music and Film downloads and some high cost software like Photoshop CS4 rather than a Free Limited life product like Windows 7 (beta or any subsequent evaluation / test build).
Downloading stuff like SUSE Linux from Novell where Novell actually encourages the use of the Torrent mechanism is 100% legal and fine.
Now you might have other constraints in that your access to the Internet is granted only that you DON'T use P2P sites (not sure how enforceable that would be in a Court of Law) or exceed your download limit (if you have one).
There could be other restrictions such as you aren't allowed to use the Internet in such a way that it severely degrades the service to other users.
So using Torrents is certainly NOT illegal -- but your activities Could be.