The #1 feature of Bamboo for me is the integration with the rest of the Atlassian suite of products. It's essentially a simple, one-click add-on to my current infrastructure, which consists of JIRA, Bitbucket & Confluence.
Setting up any number of build plans in Bamboo was super easy & quick, and integrating tests into those builds was also easy (and very useful). It helps us keep our manual QA costs down.
Also - as with all Atlassian products - it's cheap. :)
As with a lot of the Atlassian products (in my opinion), it's sometimes hard to get a good answer to questions for things. For example, i don't think the overall documentation online is the greatest - I don't always want to reach out to a support person for answers.
Getting build automation into our team's workflow was our main goal, but creating/automating tests was integral in freeing time that we were spending on manually QA'ing various parts of our applications.