Here's a serious thought that may or may not be helpful if you're still considering ventilated seats, but it really has nothing to do with long term experience with them in an M-B.
When we were preparing to purchase our 2014 212, I thought it would be nice to have the multi-contoured seats, so we looked at a car with that option. This car also happened to have the ventilated seats. Like you, we have the ventilated seats in our Lexus, but I never found them to be particularly beneficial, but since they were already there, I was willing to accept them as the car had all the rest of a long list of packages and options I wanted.
When I got behind the wheel for a test drive, however, I immediately noticed how rock-hard the seats were, and I was uncomfortable for the entire, albeit short, drive. During a follow-up conversation with the salesman, he did mention that the seats are pretty firm in these cars and, over the next week, I was able to find several threads related to complaints about uncomfortable seats. I had just about given up on my plans when I thought I would try again by going to a different dealer, talking with another salesman, and testing another car.
This time, however, I tried a car without the multi-contoured seats. As it happened this time, the car I drove also lacked the ventilated seats, but after the Lexus experience, I didn't particularly care about that. Unlike my first experience, this test drive seemed perfectly acceptable, so we went about finding a car with all the features I wanted excluding the MC seats, even though I had no real proof whether the discomfort could have been caused by any under-seat equipment for the MC seats, the vented seats, both, or neither.
The point that I'm making is that if you do eventually make the decision that you want the ventilated seats, you should be very careful and sure that you test drive a car with ventilated seats to insure that they don't contribute to any seat discomfort as far as you're concerned.
I do wish you Good Luck with your search for additional testimonials about the benefits of seats with a water vac!