People keep mentioning QNX and VxWorks as if there are no other popular microkernels.
There is another commercial microkernel, EKA2(derived from EKA morphed from EPOC) used as a base for Symbian in current/upcoming generations of Nokia and Sony Ericson smartphones. Given that the phones run on low power CPUs, symbian performance is pretty good. Since it is a microkernel the current EKA2 generation has an extensive(and simple) security model where every process is marked with a privilege such that security-crucial servers can easily ignore IPC from servers deemed to be less secure.
This feature of isolating various processes in the system without serverely restricting the APIs would be very difficult to model in a monolithic kernel.

Additionally because Symbian is based on a microkernel architecture, there is very little coupling between system components, so one can replace the filesystem, ip stack, etc, without recompiling the kernel avoiding the need for vendors to maintain giant patches and having to fix them with every new release as is done with linux.