I work at an organization that provides web software as a service, basically as the company's whole QA department. It's not unusual for me inspect the CSS to try and diagnose browser issues and speculate on solutions. I'm no CSS wizard, but I can usually diagnose enough basic issues to save the developers' time. In my case, though, I just download a local copy and edit the CSS files in the directory, as I need to test browsers without style sheet override addons.
I will say, though, that things like stylish are probably a great way to learn how CSS works and 'real' sites are put together.