I know this is an older thread but we use them as a resource, so...
We just put in the Grundfos comfort system, based on some comments I had seen from Jim in another thread. I had re-insulated the pipes everywhere I could access them, but there is still about a 60-foot run through an inaccessible area where the water cools when not in use. It then runs into a crawl where everything is now well-insulated. It goes into the downstairs bathroom and then up a chase to two second-floor bathrooms and finally to a dressing area with separate sink. That's where we put the valve. In the other direction from the water heater is the kitchen, a wet bar, dishwasher, the laundry room and an upstairs bathroom at the end of the run. That's where we put the other valve. We had basically become used to having to run the hot for an extraordinary period of time just to get anything but cold, so we just lived with the cold water coming from the hot unless we really needed the hot, say for a shower, and then you just let it run while doing something else for a few minutes until you got the first hints of warm water. Major inconvenience and water waste. No practical way to run a dedicated return line without an extraordinary expense. However, because the runs are so long, and the systems that return through the cold line only recirculate a trickle of water, any demand-based system would be worthless. So a system that is running a dribble most of the day seemed like the best option, and that's what Grundfos's system offered with the heater-mounted pump and valves at the end of each run.
I have to say that the system works very well.
Even in the present hot summer, we have absolutely no issue with the cold side being hot. Because there is a lot of pipe between the sinks on which the valves are installed and the other fixtures, the cold side only gets a little warm in the vicinity of the sink on which the valve is installed, and it's gone in a moment even at that sink. As these are both lesser-used bathroom lavatory sinks whose use in any event always involves hot or warm water, one wouldn't even notice that some warm is coming momentarily from the cold side at those sinks. The cold side is cold everywhere else. But the quality of life improvement is enormous. Now, on the side with the very long run, you can step into a shower or run a bath, and just open the hot side, whereupon you get warm water in less than three seconds; you can step right in. The water that previously would go down the drain during the interminable wait before you could even step into the shower, is replaced with immediate use of that water. It takes a while for the water to get really hot on the end with the long run, but that's okay; one just starts to temper with cold as one feels it warming up. At all sinks, it's a transformation: we can wash hands in warm water, shave in warm/hot water, etc.,without having to waste time and water waiting for some warmth.
On the side with the shorter run, where you still had to spill a lot of cold from the hot before, say, running the dishwasher, now all you do is rinse dishes with a little from the hot side or wash hands after loading the dishwasher, and you get a quick move from the instant warm to the truly hot so you can start the dishwasher. And when you fill a glass of cold water at the kitchen sink, it's truly cold because the crossover valve is a long way from there and the water has cooled by the time it returns past the kitchen sink.
This system has really improved our quality of life and allowed us not to have to waste water or incur a substantial capital expense just to have the comfort of warm water in less than 3 seconds after opening the tap. I understand our oil bill may rise a smidge, but I will keep an eye on it to see if it justifies some additional capital expenditures. As I grow to understand how the Grundfos system works in practice, I'm confident that any increase will not be material. Accordingly, I'm only sorry that it us so long to find out about it from this forum. Thanks, Jim!