BitTorrent 101: A Peer-to-Peer Primer
Getting Down to the Nuts and Bolts of BitTorrent
http://cws.internet.com/article/3989-.htm
When using BitTorrent, it's important to keep in mind that while downloads can be fairly quick for popular files with lots of seeds and peers, they can be quite slow for less popular files with relatively few of them.

In some cases, BitTorrent downloads can often take many hours or even days to complete, irrespective of how fast your Internet connection is. Also, BitTorrent download speeds are typically not constant — they start out slow and increase in speed as more peers connect to each other and the swarm grows.

Another potential issue to be aware of is that some ISPs (Comcast is reportedly one of them) block, filter, or otherwise interfere with BitTorrent traffic on their networks. These measures generally don't prevent you from using BitTorrent, but they can slow transfers and cause other problems, such as overall reduced performance of your Internet connection.

Last but not least, when you use BitTorrent or any other P2P file-sharing technology, you will find plenty of public domain material available along with lots of copyrighted content available for download. Remember that just because something is there for the taking doesn't mean it's legal to take it.