The first couple of series were genuinely very good, and the third was just OK. As someone who can't stand Doctor Who, it was basically the show I always wanted it to be, with its more serious tone and Cumberbatch/Freeman having an excellent curmudgeonly Doctor/audience surrogate companion dynamic respectively.
I'm British, and it became actively annoying how long it took to produce new episodes. 13 episodes in 7 years, and after the second season the quality deteriorated sharply. Ridiculously low rate of output when compared with how quickly teams of American writers can put out an entire season of a show with more total episodes than that in a year. I know the UK has a different TV auteur writing tradition, and Gatiss/Moffat had to work around the busy Hollywood schedules of Sherlock's stars, but the quality eventually wasn't there to justify the short seasons and long gaps. Not to mention the show was largely based on existing stories.