Since everyone is touting all the hazards I will assume by now everyone understands that, yes it is a bad idea, of course no one should do it ect. However your question wasn't should you, it was has anyone. So risking being called out I will admit that yes I actually have experimented a little with one but the results were a bit disappointing, especially because I was terrified that I was going to go blind in the process.
The only cool things I was able to do was to smoke some paper towel and cardboard and make a fluorescent light glow. I wanted to see if the waves could be guided but shooting the waves down a metal pipe proved useless, the energy just dissipated. I measured the strength of the waves by placing a fluorescent light tube in front of the maser. Apparently you can focus the beams if you can use fancy math stuff and create a horn of the proper dimensions but no easy button was found.
I just couldn't come up with some productive or destructive applications for the maser. sure if you hit someone's eyes it may prove dangerous otherwise you will just probably slightly warm the temperature of the object. I quickly realized microwave oven maser waves are not focused at all, the reason is that the waves need to bounce around the microwave from all directions to more evenly heat the food. So for instance a florescent tube an inch from the maser will light but if you pull it say six inches away it will almost be dark.
More observations: Even if you had a super high power maser and were able to properly create a horn to focus the beam it is a poor choice for doing anything. Metal will reflect the waves like a mirror so in the example of someone shooting the maser at a car, the more likely result is that the waves could hit the car door and be reflected back to the user instead of hitting the intended target. Maser waves pretty much just excite and heat water molecules so pointing them at electronics doesn't really do much, some heat can be created. Living things like plants and animals can be harmed though. Maybe the real application is to make an extremely dangerous and slightly ineffective hedge trimmer.
I would like to offer some encouragement however. There is nothing wrong with curious experiementation so long as saftey precautions are adequetly taken. The history of discovering the atom is rich with stories just like these, otherwise we wouldn't enjoy all the luxurios spin-offs our society takes for granted. Unfortunatley, the dangers of radiation are severe and not always immediate; many of those famous laureates died premature deaths as a direct result of their life's work and contribution. Let us learn this from this too; We, I- don't want to see anyone get hurt.
I would like to offer some encouragement however. There is nothing wrong with curious experiementation so long as saftey precautions are adequetly taken. The history of discovering the atom is rich with stories just like these, otherwise we wouldn't enjoy all the luxurios spin-offs our society takes for granted. Unfortunatley, the dangers of radiation are severe and not always immediate; many of those famous laureates died premature deaths as a direct result of their life's work and contribution. Let us learn this from this too; We, I- don't want to see anyone get hurt.