I've updated JKstat to version 0.08.
It's getting better. The accessory widgets have been cleaned up and a couple of new ones added (distribution of packet sizes on bge interfaces, and dma transfer rate on ifb graphics cards). Rates are now accurately computed based on the actual snaptime, rather than approximately based on the intended refresh interval. A couple of internal changes streamline the whole system. And I've fixed it so that actually enumerating the kstats doesn't blindly read all the data, which improves performance.
With these changes, I'm much happier that it's closing in on its design goals. I was tempted to bump the version up to 0.1, but that would probably be premature based on the number of bugs that I introduced and fixed recently.
My next idea is to build a graphical iostat. Why is this of value? Well, pictures tell you a lot - the eye is very good at interpreting graphical data. You can dynamically hide uninteresting data, or expand areas of interest for a finer view (for example, you could dynamically expand a disk's I/O to show partition data). You can show historical rates, and generally have multiple views of the same data. You can use the gui to show additional context-sensitive data beyond the basic I/O data. And you could, in the future, link to other areas of functionality - such as dtrace to show what was causing all that I/O in the first place.