uTorrent has become the number one bittorrent client because it does not attempt to do too much.
It covers all the basics of bittorrent, but uses tiny amounts of memory (944K on mine) and disk space (.21MB).

There is not much downside to the software itself. I visit many bittorrent forums and rarely see any issues with uTorrent itself.
The number one issue is getting its network status cleared (as it is with all bittorrent clients), which involves forwarding the uTorrent port through the firewall and router.

Bittorrent itself has some issues with fake releases and rarely with viruses. Bittorrent accounts for around 40% of all internet traffic and most of the content is being illegally distributed. That is why groups who represent copyright holders patrol torrents of copyrighted material and try to catch people in the act. The US statutes allow for a large penalty per item in civil court. Judgements, or settlements, end up costing users many thousands of dollars.
Steve