My 2021 R 1250 RT didn't come with ACC but I've just had it retrofitted and like you, I absolutely love it!I have it on my 2022 R1250RT Triple Black and absolutely love it. I also have it on my F150 and it is a wonderful technology on it as well.
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Interestingly, a lot of the Youtube vloggers when first testing the brand new (2021) 1250RT with the then brand new to motorcycles, ACC were very sceptical about it but generally changed their minds and came away convinced it was a useful option after trying it.
Such a shame the K1600 is not going to get it, probably the best tourer in the BMW lineup and no ACC option, that is not good.
I'd go further and say that BMW should also be offering on their premium models "blind spot monitoring" by fitting a rear radar sensor like Ducati has done.