The problem I've faced: UI becomes seriously slow and unresponsive when I have more then like 3-4 active seeding tasks for a long time. Same for leeching. Also, if I stop downloading task of some big file - UI freezes (probably until data fully flushes onto disc). And for me it's a problem which I did not face with other clients.
Windows 7 SP1 x64, latest updates
Intel i7 4770k @ 4.4Ghz
16 GB DDR3
100MBit Full Duplex connection (Up to 1Gbit to peers from my country)
I use 1GB of disc cache + enabled disc writes caching in windows.