Rating by season:
Season 1: 4 stars
Season 2: 2.5 stars
Season 3: 1.5 stars
Honestly, season 1 was great. I haven't read the Sherlock books, so can't comment from a purist perspective, but it had a good balance for a prime tv show - comedy and drama with a good heart and likeable characters and an interesting overaching storyline. The focus appeared to be more on the characters themselves rather than the mysteries they solved, which in my opinion, is a good way to frame a show.
Seasons 2 lost a lot of that. Sherlock wasn't as quirky. There was no story arc, only individual episodes, with more focus on the not-so-interesting mysteries rather than the main characters. There were some odd storyline choices that seemed to be an attempt to add drama. It felt like it was written by an almost entirely different group of writers who were writing simply to get their paycheck.
I'm only 3 episodes into season 3, but it's showing the same problems as season 2. I'll push through for now in the hope that things will get better. Presently it's more of a background noise show, unable to hold my complete interest.
I often wonder how shows end up going in such different directions, and whether the writers look back on them and wonder about that themselves. It's one thing to try and re-invent a beloved book series, but it's another to make it bland and unmemorable. I'm certainly of the opinion that, if you're going to do things different, then make sure you do it well. Crime solving shows can be entirely acceptable without needing to attach the name of a well-known book character.