Full disclosure: I used to work at UserVoice. But I had no obligation to use it at my next job, and I had to take it through the same assessment and pitch to my executive team as any other tool. We chose UserVoice, hands down.
Why? UserVoice doesn't just fit the feature requirements of a customer service tool - it's obsessively focused on the customer experience, trying and succeeding at making the customer service experience BOTH efficient and easy for everyone involved.
Add on top of that an incredibly innovative feedback system that truly helps you prioritize what customers want what? You've got a seriously badass (and affordable) resource for keeping your customers happy.
I'm very proud of my time at UserVoice and will continue loyally using their tools for many years.