From what i remember, the original BT client (mainline) really sucks poo poo. But that's  years back when i tried it. So it's probably far better now.

But i can tell u that back then, it pretty much just has 1 input, where u input ur torrent, and it downloads. Doesn't tell u much info, no other options at all, no download management, like setting priorities and all that stuff, only single download, doesn't resume next time u start it, u actually have to reload that torrent, and select the correct location and do a peice check to continue...

Basically sucks so hard i gave up using torrent in favor of limewire back then... Then i discovered Azureus, at the time when that and BitCommet had the monopoly, then i realised that Azureus took too much memory, then i discovered uTorrent, which is almost as advanced as Azureus, but took way less resources.

uTorrent was still relatively unpopular back then when i started using it, but now it seems to have grown WAY more popular (like 5x). Azureus popularity seems to have halved since, and BitCommet remained the same. Other clients like ABC and Bitlord and stuff never became too popular, but is seen from time to time. Mainline is also seen from time to time.