Never heard of them and they don't pop up on Google like this site does when you input a question.
Went and looked. Saw a useless discussion on the Grundfos Comfort System like I have had for about 2 years in the country. Poster wanted to know the pros and cons. Mostly a bunch of ignorant comments and a few comments like, "I've heard they can be problematic." Useless. Now if they solicited input from regular folks like me, I could give them chapter and verse about it because it's always in use and was just tested by a very cold Long Island winter. It isn't enough for a bunch of guys to weigh in with "haven't had any callbacks on the four I installed" or "I heard you have to install it according to the directions". An actual user could tell them that in the coldest weather, in a long, long run from hwh through a mediocre-ly insulated chase, that previously took a good minute to produce even warm water, there is now warm water promptly at every faucet in that area of the house; that if you install the 4-way valve truly at the end of the run (by figuring out where it is), every faucet previous to that valve (shower, lavatories, everything) will have warm water within a couple of seconds of opening the tap, and hot water not long after. Washing your hands? You will have warm water to do it with. Shaving? Washing face? All things that don't use much water and thus it isn't worth running and running the water just to warm it up to use. Now you have warm water to do it with. Showering? Warm then hot water very quickly. After 40 years of washing hands and face with cold water in that main bedroom area of the house (with master, kids, and guest rooms), now it feels positively luxurious to have warm water to do it with, even when it's 7 degrees outside, and even when you're the first consumer of water in the morning. And on and on.
Here on this forum, you'd have a complete picture. Over there, it's a bunch of self-important types who have little to contribute because you don't hear anything from the end user.