KTorrent is definitely the most full-featured BitTorrent client for KDE. It is complete by all means and highly configurable, offering, among many others: search capabilities (with the results opened in a new tab and the possibility to add custom search sites), the possibility to create torrent files, pause/resume torrent downloading, system tray integration, detailed information about the torrents it downloads, a queue manager, various useful plugins. KTorrent has come to be probably the most complete BitTorrent client out there, being also fast and having an option to use low CPU resources.
Like most of the KDE applications, you can configure the shortcuts and control KTorrent through the system tray popup menu.
Currently the stable release for KDE 3.5.x is at version 2.2.7, while work is in progress at KTorrent 3.1 (release candidate 3.1rc1 is out). The official website offers a FAQ and a forum, so getting help won't be hard in case you really need it.
Official KTorrent Website
Deluge is a BitTorrent client made in GTK, coming with many features, being in my opinion KTorrent's counterpart in GNOME. It is expandable through many plugins, like search for torrents (but unfortunately it opens the results in a browser window instead of in the application itself), or the Create Torrent plugin.
The interface is a little inconsistent, meaning some widgets change their size depending on the text inside them. When this changes, it can get a little annoying. It also has system tray integration, can display detailed information about the torrents, it has options to limit bandwidth for upload/download speeds, if you really, really need it. Deluge is probably the best BitTorrent client for GNOME.
Official Deluge Website
Transmission is a light client written in GTK. It doesn't feature as many options as KTorrent or Deluge, but it's lighter and has a simple interface. Some of the highlights: ability to pause/resume downloads, system tray integration, bandwidth limiting settings, detailed information about the torrents it downloads. A small, yet enough powerful client for GNOME.
Official Transmission Website
All the three clients do what they are supposed to do: download torrents. If I am to recommend a single one, I'd go for KTorrent, but Deluge is a very good and powerful client as well. Actually, this just might be a matter of taste. If you like more GNOME applications, go with Deluge, if you are a KDE fan, than KTorrent is definitely for you. If you are looking for something lighter and with a somewhat cleaner interface, then Transmission might be just the right choice.
Note: Full reviews for KTorrent and Deluge can be found here and here.
Updated: Jun 14, 2008 (Created: Jun 13, 2008)