Recs.
Updated
SpecsUpdate
Pros
Pro Full-featured
µTorrent covers basics such as Prioritization, Selective Downloading, DHT, PEX, LPD and Magnet URIs as well as Broadcatching, Sequential downloading and Search. It includes MSE/PE for security. It has graphical and web interfaces. Has UPnP, NAT-PMP, NAT traversal for automatic router configuration. Has UDP tracker and µTP data transfer protocol support. Even covers IPv6, web seeding, Embedded tracker, Super-seeding and proxy servers. And it auto-updates.
Cons
Con Bundles bloat
Installation wizard starting with version 3 offers installing a variety of garbage features including toolbars and ads. And makes the process of opting out of such bundled software difficult. It's possible to find version 2.2.x online (1,2) - same lightweight, feature packed application without all the unneeded extras or ads.
Con Inappropriate ads (can be disabled)
Not only does μTorrent have ads in the free version, they tend to be inappropriate ads for scammy things like "meet russian women".
Ads can be disabled in the options. It's important to note that disabling ads (as well as all bundled software and featured content) can be a difficult process as it requires manually editing multiple key/value pairs in advanced settings.
The following keys have to all be turned to false by going to options > preferences > advanced:
bt.enable_pulse
distributed_share.enable
gui.show_notorrents_node
offers.left_rail_offer_enabled
gui.show_plus_upsell
offers.content_offer_autoexec
offers.sponsored_torrent_offer_enabled
offers.featured_content.badge.enabled
offers.featured_content_notifications_enabled
offers.featured_content_rss_enabled