The general opinion is that it works for text, but it doesn't seem to be all that robust with images or graphs. One tester reported graphs that were to small to be usable, and another reported problems with image positions.
A couple testers were bothered by the fact that the plug-in only worked with ID CS4 and ID CS 5, but not earlier versions (this was buried in the documentation and not stated clearly). There were also 1 or 2 reports of problems installing it; it tended to ignore CS4 if CS5 was also installed.
Liz Castro worked up a list of supported features: font size, style, special chars, text alignment, para align, cover, links, lists, TOC, color, tables. But there was also a comment that some page breaks were lost, and that when working from multiple files some formatting was lost.
All in all, it sounds like a good addition to inDesign, but I'm not entirely sure it's better than working with MobiCreator and KindleGen.
P.S. I'd like to thank @lizcastro, @crych, @sos_jr, @adamjury, @Timeroler, @mikecane, @JoaoCP, and others for trying the plug-in.