uTorrent is one of the most popular BitTorrent clients around, and for good reason. It's lightweight, relatively simple to use, and it supports all the major platforms.
Windows users have always had a great experience with uTorrent. It's never been a resource hog, and it's always been reliable. Mac users have had a less consistent experience. The first few versions of the Mac client were buggy and unreliable, but the latest version is much better.
The biggest selling point of uTorrent is its simplicity. It's not loaded down with features like some of the other clients. It has everything you need to get the job done, and nothing you don't.
If you're looking for a great BitTorrent client,uTorrent is definitely worth a try.