I have tried importing bugzilla into 2.3 (since 2.4.1 being broken is a known bug).

I have problems with the following:

• most web browsers/proxies seem to timeout on a request that takes this long (an hour or more with a large bugzilla database). To get around this, I do a GET request on the bugzilla import using a tool that controls/ignores the timeout, and send the JSESSIONID cookie so it will log in.
• I get the following exception, presumable because the session timeout is less than the length of time it takes.
  [10:09:56] java.lang.IllegalStateException: getAttribute: Session already invalidated
  at org.apache.catalina.session.StandardSession.getAttribute(StandardSession.java:954)
  at org.apache.catalina.session.StandardSessionFacade.getAttribute(StandardSessionFacade.java:171)
  at webwork.action.factory.SessionMap.get(SessionMap.java:86)
  at com.atlassian.jira.web.action.JiraWebActionSupport.getRemoteUser(JiraWebActionSupport.java:51)
  at com.atlassian.jira.web.action.util.BugzillaImport.createChangeHistory(BugzillaImport.java:335)
  at com.atlassian.jira.web.action.util.BugzillaImport.createIssue(BugzillaImport.java:271)
  at com.atlassian.jira.web.action.util.BugzillaImport.createIssues(BugzillaImport.java:210)
  at com.atlassian.jira.web.action.util.BugzillaImport.doExecute(BugzillaImport.java:132)
  at webwork.action.ActionSupport.execute(ActionSupport.java:116)

I would suggest that the bugzilla -> XML process should be a separate Java application, and this XML should then be imported via the regular JIRA mechanism.

I rate this as major as it could potentially push our company's decision to adopt JIRA over our existing bugzilla installation into the negative. We are currently evaluating it.