Question Ranking
Pros
Pro Easy to use UI
Pro Huge variety of OSs, browsers and devices
There are over 200 desktop/mobile/tablet browser-platform combinations possible.
Pro Good selection of developer tools
IE, FF, Safari, Chrome and Opera come with additional web development browser extensions installed, such as Firebug or YSlow.
Pro Free JS unit tests for open-source projects
Developers are advised to use the contact form. The JavaScript API is free for open source projects. Unclear whether automated testing with Selenium is also free - apparently not?
Pro Selenium automation integration
Pro Get screenshots across many devices
Can request screenshots from many devices for comparison.
Pro Integration with CI services
Pro Might be able to test on real iOS devices
It's not very clear if real iOS devices are available for automated testing.
On one hand, https://www.browserstack.com/mobile-browser-emulator is all about emulators for both Android and iOS.
On the other, https://www.browserstack.com/automate says
"Test across real iOS devices and Android mobile emulators"
Then again, https://www.browserstack.com/list-of-browsers-and-platforms?product=automate lists "real iOS devices".
Pro Run concurrent tests in parallel
Open source project accounts are limited to at most two concurrent VMs.
Cons
Con Real Apple devices are unreliable as of August 2015
Real Apple devices are a new feature that BrowserStack has introduced. At the moment (August 2015) there are unreliable. Excerpt from an email reply from BrowserStack support:
we provide real iOS devices (Beta) and iOS simulators for Selenium browser testing on Automate. Since these devices are in beta, you might encounter a few problems while running tests on them.
To avoid this issue, we would suggest you try running your tests on iOS simulators.
Con Sessions are rejected instead of queued up when all VMs are in use
Con Uses Android emulators for mobile testing
Con May be unreliable
The same test may succeed for some users and fails for others. Apparently there are problems with the test client running on Windows.
Con Security incident in 2014
Con Re-use of VMs across different customers
Con No Linux support