Azureus has been stable and reliable for years, but for some reason started refusing to launch on Ubuntu last week (something to do with libgobject and Java). Luckily, KTorrent has arrived. The version in Feisty (2.1) is quite buggy, but v2.2 is excellent and has all of the features that made Azureus so useful, as well as having a fast UI and being easy to configure.
Also of note: Amarok had a new release last month, with lots of small improvements. As with KTorrent, it's worth downloading the Ubuntu/Kubuntu .deb file for each app rather than waiting for the next 6-month Ubuntu release cycle.