I just have one that came with a bushnell, found it awkward to mount it and then remove it and mount again then remove when looking for game around large open terrain. Binoculars are just so much quicker to use.
I'm not really sure what the best use of them would be, pain in the butt if the weather is really cold or really hot, or the mosquito's are thick, plus you can't swap back and forth to other windows any quicker, and you kind of have to pivot your body around them as they swivel point is at the window, perhaps if your only spotting game in one direction with say trees to one side of you it may work, but for me they just seemed kind of cumbersome and I have no interest in bringing it out with me for a spotting scope looking for game again, just my experience though.