Was surprised to see this ad in my Gmail today:
I usually get a lot of Linux and embedded hardware related ads because of the OpenMoko mailinglists, but Minix 3 was really a surprise. I tried running it in VMWare about two years ago after reading a nice introduction to it on OSNews. Can't say I was impressed. It sure didn't look like much back then (and still doesn't, it's just X), but microkernels are very interesting.
There are a lot of L4 kernel based operating systems, and a few of them even run on the Neo1973(Video from here). Hopefully this one will too some day.
There will always be some overhead when using a microkernel compared to a monolithic kernel because of the extra IPC, but the modern implementations are not that far behind. It will be interesting to see how useful the different versions will become, although there's little hope some of them will ever be completed...