I guess you like it or you don't. Certainly young white American kids were not playing blues in any great numbers before that happened. Jeff Beck recalls being very mystified that US audiences took the Brits seriously in the early days. However, time has passed and British blues has a pedigree which is only a few decades shorter than the US has. Certainly the Brits were taken very seriously by some of the US giants - BB King and Muddy Waters in particular. British urban blues is as authentic as any other type. It now has firm musical and historical roots and has definitely contributed to the development of the genre. Whether you like it or not is another question. I would say the Stones have not been a serious blues band since Mick Taylor left in 1974. In fact their heyday as a blues band had already come and gone before that.
I guess they are a bit gross at times, but there have been occasional bursts of lyrical and musical intelligence. Notably songs like "Wild Horses" and "Waiting on a Friend" prove that they really can write grown up songs about women if they want to. "High Wire" and "Undercover" tell pithy political truths, even if they are not actually musical masterpieces. I think they could have done better both musically and lyrically had they really tried.
At the end of the day, I like them. I grew up with them and they feel like an old gang of friends. I know they are pretty imperfect, like lots of my other friends. They have never really done any serious harm that I can see, however, and I can see the joke when they are taking the piss. I don't expect too much of them, so I guess I don't get too disappointed!