New to this version is process monitoring, better temperature and fan support in G4 Powerbooks (like mine), and a disk activity graph that let's you monitor disk performance over time. For keeping an eye on what's happening under your Mac's hood, there's almost nothing better, and all the iStat stuff is free.
Of course, if you do use one of their apps, make sure to send them some cash to help them keep up the good work. Always support good software, people. Just like NPR, if you don't support it, you'll lose it.