I really love Jumi, there hasn't been a project where I didn't find it useful.

But I have 2 suggestions on how it may even be better.
First one:
Replace the basic textarea used for inserting code with a real code editor there are 2 really good Javascript code editors that would improve usability of Jumi a lot.\

Second one:
This may be a bit harder to accomplish, but how about adding parameters to Jumi plugin, in example we want text to use sIFR, or just that one image to have a fancy border. So that plugin would not just add some code to the page but to be really dynamical and add code according to parameters set.

This is a great extension doing, just what a lot of people need. Simple to use, just like Joomla's Menu module, great addition is possibility to disable list bullets for those not that familiar with CSS. Maybe in future versions there could be added a hover image support?