It's nowhere near as perfect as utorrent once was. But it's pretty good and I have plenty of extra memory (it's currently using 305mb while seeding 40+ torrents, dropping down to 65mb if I pause them all.)
I wouldn't be surprised if qbittorrent predates uTorrent; Wikipedia says it dates to 2006. But it wasn't popular on windows until uTorrent started doing dodgy things.
uTorrent used to be really efficient, small memory-footprint, full featured bittorrent-client. One of the best software I've ever used to download... perfectly legal content [1] from the internets.
Now it's full of ads and performs poorly.
[1] Like all the different Linux distro install images over and over again.
It fits the efficient, small memory-footprint and no ads requirements. "Full Featured" is subjective as it depends upon what you consider "Full Featured".