Powerful, easy to use,fantastic customer support.
Right now my favorite feature is the 'inline' FAQ whereby I can add a 'What's This?' icon anywhere on my site which then pops up the UseResponse widget with an FAQ related to the user's current asset. This is fully integrated to the support system allowing me to both inform users of features interactively and allow for extended/related answers and even customer support.
I tested a number of popular/leading competing services and found UseResponse to be the most comprehensive and easy to use of all of them and the price is decidedly right. For me the final deciding factor was that a company that creates customer support software in fact provides superior customer support too. I was able to learn about and set up all the features I needed with direct and caring support.
I don't often 'gush' about software (actually usually I gripe :) ) but UseResponse IMHO deserves it.