We use UserVoice for Handcraft. Before launch we looked around at various tools and were ultimately looking for something that was quick and easy to integrate, relatively unobtrusive, and really easy to use. Uservoice meets those goals because you can start voting straight away, it has sign-in integration with services like Twitter, and you can hook it into your site with one of those overlay tabs ("Feedback" lip on the left of the screen).
The quality and kind of feedback has varied. Since Handcraft is a technical tool aimed at designers, we sort of straddle two groups of users: the front-end developers who report technical bugs and features like "I want zen coding" or "pasting a certain kind of code leads to this error", and interaction designers who're looking to improve their workflow with feedback like "please add FTP support". But if your tool is more focused on a single group then you might have different results.
I find it's been useful to have Uservoice because it's been another place where people can go to drop some feedback and, more importantly, vote on others' feedback. That helps us prioritise where people want the tool to go next. Zen coding has been a feature request for a year with several upvotes, and we recently integrated it so we could go in there and change the status to "completed", which felt pretty good. ;-)
It's important that this is just one avenue of feedback, however. We also make it easy to email us, tweet at us, and have a custom feedback flow built into the first use experience so early questions can be quickly answered. So don't depend too much on it as a holistic solution.
Once Handcraft gets bigger we may move to Get Satisfaction. I think the biggest difference between something like Uservoice and Get Satisfaction is that the latter allows people to help each other. So if you have a community of users and you want to decentralise customer support a bit, Get Satisfaction is a great tool. For simple feedback purposes, you'll probably do fine with Uservoice or comparable tools.