Ranked in these QuestionsQuestion Ranking
Pro Provides the option of running client/servers either separately or on the Development computer
These options are really helpful in cases, where you have the luxury of multiple machines (tablets, mobile phones desktops) around you.
Pro Provides both Browser based testing as well as headless tests
Karma eases out the UI testing process as you can test your code on all your devices let it be smartphones, tablets or your very own desktop. If you don't want all of that, you always have the option of headless testing using a PhantomJS instance.
Pro You can test your code in cross browser environments
Being able to test your code directly via your testing tool is a breeze! You don't need to download a fancy tool to see how your app looks in a number of different browsers, now that Karma would do the job for you.
Pro Easily extensible
Do you prefer other test frameworks such as Mocha, Jasmine, qUnit or any other framework? Well you're in luck as Karma can be easiliy be extended to wrap around ANY framework of your choice.
Pro Has plugins for WebStorm and is supported by the Netbeans IDE
Thanks to Karma, you won't need to spawn up a new terminal just so that you can test your app, you can now code and test right from the IDE
Con No Support for NodeJS testing
Currently Karma doesn't support testing of apps built on NodeJS. So if you have a node app, you don't want to use Karma, Mocha or Jasmine can do the job for you.
Con No plugin for Eclipse (yet)
Do most of your code using Eclipse, well, you're in bad luck. Karma doesn't have an eclipse plugin, though if you are a real die hard eclipse fan, you can see this little hack to be able to run Karma from inside Eclipse Link
(Time of writing: July 2014)