Ranked in these QuestionsQuestion Ranking
Pro Very minimalistic, straightforward design philosophy
Transmission is great for getting things done without looking too much under the hood. It still has advanced options, they're just tucked away from sight.
Pro Free, ad-less and open source
Transmission is free, ad-less and open source software licensed under GPL with parts under MIT with source code available here. Being open source allows anyone to check if the software has no malicious code, is secure and respects privacy.
Pro Native Mac, GTK+ and Qt interfaces provide seamless integration with OSX and Linux systems
Transmission works well on OSX and various distributions of Linux due to having interfaces natively developed for those operating systems instead of haphazardly ported.
The Mac interface is written in Objective-C, uses dock badges and Growl notifications while the GTK+ interface follows GNOME Human Interface Guidelines and has support for popup notifications, desktop sounds, and a system tray.
There's also an unofficial port of Transmission-Qt for Windows.
Transmission covers basics such as Prioritization, Selective Downloading, DHT, PEX, LPD and Magnet URIs. It has graphical, web and command line interfaces. It includes MSE/PE for security. Has UPnP, NAT-PMP, NAT traversal for automatic router configuration. Has UDP tracker and µTP data transfer protocol support. Even covers IPv6, Web seeding and Local Peer Discovery. And it auto-updates.
Pro Very lightweight
Transmission-gtk uses only 17 mb of RAM in idle state and 24 mb in download state.
Pro Has a command line interface and a daemon
Pro Pre-installed on multiple Linux distributions, including Ubuntu
For those who prefer Linux, the Ubuntu OS comes with Transmission already installed, so you can get your torrents up and running immediately.
Pro Works great on a seedbox with its web interface
Transmission has a web interfaces that can be used to remotely control torrents. It's great as a seedbox (a dedicated private server for uploading and downloading content) due to built-in webseed support.
Con The app is hiding too much information
While the app is clearly going for minimalism, some users might feel that the default window that consists of just torrents and their progress bars might provide a bit too little information or functionality. For example, even accessing file lists requires opening up a separate window and navigating to a specific tab.
Con No embedded tracker
An embedded tracker allows turning the client into a server offering an easy way of sharing files with others. Transmission lacks embedded tracker support so another method of sharing or different client has to be used to share files.
Con No preload magnet metadata
Transmission is unable to preload magnet metadata.
Con No proxy server support
Con No tracker exchange
Tracker exchange functionality allows peers to exchange information about trackers of a given file. It is another way of ensuring that the file is downloaded from the optimal source. Transmission lacks this functionality.
Con No super-seeding support
Super-seeding is an algorithm used to allow downloaders become uploaders quicker. Transmission lacks support for this functionality.
Con No broadcatching support
Broadcatching is a method of automatically downloading content as soon as it's available. Broadcatching in BitTorrent clients is usually implemented via RSS. Transmission requires manually selecting each download.
Con No built-in search engine
Built-in search engines allow finding torrents to download right from withing the torrent client. Transmission lacks such functionality.