Question to OP: what do you feel is not slick or does not have a professional feel about Calibre's UI?
The main reasons people (and more typically in these days of free software, firms) go for paid software are
a. stability
b. support
Calibre is stable - it has been around for a while and is used on a regular basis by a very large number of people. In terms of support, nothing can beat the kind of customized support one can get here. The original creator of the software, along with a bunch of other experienced users respond to questions and more promptly than you can imagine. Plus you don't have to be on hold on the tech support line while you wait for the next available agent and don't have to go through a long list of useless questions/suggestions when you do get to an agent.
