I'm not sure I understand what you mean by a client off feature. I run Mint 13 KDE. I installed Transmission because that was the first torrent program I used as a new Linux user just a few months ago. It's simple and does what I need. I can close Transmission without shutting down the entire computer. If I only have one active torrent, for example, I can hilite that torrent and click pause at any time. Transmission is still open but up and down activity is 0. I can resume at any time by unpausing. I tend to seed torrents for quite some time after my download is complete and 15x is not uncommon. When I don't want Transmission running any longer, I just pause all active torrents and close the program. My computer lives on.
Maybe you are looking for an automatic client close feature that activates when a download completes? Don't know, haven't tried.
i5 3570k, ASRock z77 Extreme 4, 8 Gb Ripjaws 1600, Antec 430w psu, HVR 1600 tv tuner, custom case- marble top, oak face. Carver & DCM Time Window sound system. Mint 14 KDE.