I personally use testFairy (http://www.testfairy.com/). Main reasons:
They require zero integration (you don't need to change anything in your code.
They record the user's screen while he is using your app - so you can see and understand better what the user did. It is great when debugging logical bugs.
You can get a heat map of where users touch your screen.
You get a matrix of all activities and devices that your app has been tested on. So you can see if you need to test specific areas in your app against specific devices and get overall impression of your qa coverage.
JIRA integration - easy to create an issue in JIRA directly from their dashboard.
They have tools that helps you with your crowd testing campaign.