The microwave bands are actually quite populated, and during contests in metropolitan areas, they can actually sound like a pileup on 20 meters. Up to 10 GHz, getting on the air is now pretty routine, and most guys build their own radios. It's not unusual to have rubidium locked synthesized local oscillators, so running SSB and CW is as stable as it is on the lower bands. Receivers are extremely sensitive, and it's not unusual to be able to hear the black body radiation from trees, rocks, and other objects. Hearing it off the moon is a bit more challenging, but if you can do it, it's a pretty good sign that your receiver is working. All of the above is routine now, and can be done by moderately technical hams with an experienced mentor.
At 24 GHz and above, it's starting to get a bit rarefied, so far as population and radio performance goes. There are transverter kits for up to 47 GHz available off the shelf. Above that requires quite a bit of resourcefulness and an eBay account. There are some hams who have their own die bonding machines, and can get samples of transistors and other devices for extremely high frequencies, and build their own radios. There is an annual convention of microwave oriented hams, and there are examples of activity up to at least 120 GHz presented at these gatherings.
Amateur television is a heavy user of microwave bands up to 10 GHz that I'm aware of, and as has already been mentioned, a lot of repeaters and remote base systems are getting linked via microwave.
