Microsoft has established a good pattern: they've been taking tools that they use internally, polishing them up, and releasing them as free tools through their web release (WR) program. This flow most recently brought us ExBPA 2.0, and now a new tool joins the family: the Exchange User Monitor, or ExMon. The cool thing (as Chris points out on the Exchange team blog) is that ExMon can both aggregate data and show you user-specific performance data. If you have a user or two who consistently complain about performance, ExMon gives you a quantitative tool to ID and fix the underlying problem. Check it out.