qBittorrent is an easy to use, good looking, and featureful
but lightweight C++ / Qt4 Bittorrent Client.

Note that from version 3.3.15 you will get a warning on startup that
qbittorrent has a new way of saving things. If you want to backup
your old config directory first, you will find it here:

~/.config/qBittorrent

That way you can return to an older version by swapping directories
around.