I've gotten one of those maybe every decade or so. I always lie through my teeth, listing the shows I wish I had watched or felt like I should have watched, with the result that I don't trust the Nielsens at all.
I was invited to get a box put on my TV, but it turns out that they put a box on EVERY television in the house, so you can't, say, watch some crappy sitcom but have PBS on your box TV. I think I recall that they have to go through the whole house to make sure you aren't keeping one of your TVs a secret. Plus there were some other restrictions that I no longer recall. It sounded like a real pain in the butt, to be honest, so I said No Thank You.