It should be said that this depends entirely on the file in question, amount of peers and speed of the non bittorent source.
You will never download faster than the maximum download bandwidth of your connection, and if you are downloading from a place with enough bandwidth, it will be faster than using Bittorrent.
However, as internet speeds are getting faster and faster, we are getting to the point where (smaller) websites and hosts can not keep up.
For example, when I was looking around at colocation prices up in London a few months ago, I was quoted absolutely terrible prices for a 5Mb connection. If I was to take this, it would serve the majority of people well.
However, on the same line, if I was to host a few 500Mb files, and everyone had Cable (50Mb) or fast ADSL2+ (24Mb) lines, you would notice that my server would not be able to serve up speed that fast to you...
... however, if I was to offer you the same file through Bittorrent and there were 200 people, each giving just 30Kb/s, that would equal 5.8Mb/s (and, many people have much faster upload speed than that!).... and now, if I was to run Bittorrent on my server and offer the same file, it would mean that there is a total of 10.8Mb/s downloadable - much more than I could provide through that 5Mb/s pipe on my own.
Bittorrent is very much a numbers game, you have to have enough people uploading with enough bandwidth... Because of extra steps involved such as integrity checking (and the fact that you should upload back), it is hard for it to ever beat direct downloading from a good site with enough bandwidth, but, for many smaller sites - it is brilliant or even larger sites who simply want to save money off their bandwidth bill.