"I think microkernels are stupid. They push the problem space into *communication*, which is actually a much bigger and fundamental problem than the small problem they are purporting to fix. They also lead to horrible extra complexity as you then have to fight the microkernel model, and make up new ways to avoid the extra communication latencies etc. Hurd is a great example of this kind of suckiness, where people made up whole new memory mapping models just because the normal "just make a quick system call within the same context" model had been broken by the microkernel model.
Btw, it's not just microkernels. Any time you have "one overriding idea", and push your idea as a superior ideology, you're going to be wrong. Microkernels had one such ideology, there have been others. It's all BS. The fact is, reality is complicated, and not amenable to the "one large idea" model of problem solving. The only way that problems get solved in real life is with a lot of hard work on getting the details right. Not by some over-arching ideology that somehow magically makes things work."