I initially chose FogBugz because I loved how easy it was to create a new issue in the system (which at the time was significantly better than the others I tried). It only requires a single line of text for the description. There are additional fields you can use, but they are never required. That said, FogBugz's UI hasn't quite kept up with the latest and greatest, so we found it to be somewhat unsatisfying and even quite annoying at times (not enough inline editing for example). Ultimately the reason we started exploring other tools was because of its slow performance (although I'm not sure if it was due to having it deployed on a Linux server rather than Windows).
We tested Jira with the GreenHopper plugin (agile development) for about a month. Most of the team made the transition quite easily (a few guys wanted to go back to FogBugz) and it was significantly faster than our FogBugz system--which made the team much happier. For our planning meetings, we all really liked the user story board view in GreenHopper, and overall I found the user interface more usable than I expected. However, Jira definitely feels like a typical old-school 'enterprisey' Java app that has been quickly updated to stay current (not to mention the sprawling feature set it has accumulated over the years). Its UI definitely lacks the cohesiveness of FogBugz (and especially Pivotal Tracker).
In the end, our company was acquired by a company also using FogBugz, so the team never had a chance to make a decision one way or the other.