We've switched to Pivotal Tracker (also in free beta) and find it much more suited to rapid development and "agile" practices. In particular, we find the predictive tools actually useful.
Personally I think Jira looks like it was written 10 years ago and can get quite slow at times, which together with the fact that there's no AJAX and every click reloads an entire page means it can be a very unpleasant experience.
I'm in love with the ability to very easily create different issue types with different attributes, states, and work flows. That lets us use the same system as a project management tool with release schedules, features, sub-tasks, reporting on hours breakdown by resource etc..., and a customer facing issue reporting/tracking system, and an internal bug tracker, etc...
edit--- Also love the great integrations with Confluence, Eclipse, SVN, Hudson, etc....
It is free for one project and unlimited users, paid ones have more space and projects starting at 12$/mo