It is an understatement to say I was pretty much disappointed after the third episode. Everything was there - well, in theory: my favorite genre, a good plot, a great cast, the famous British humor. But something was just off and I even tried watching the first three episodes again after the season finale, but no. I simply don't and can't like them, not sure why.
The fourth episode was the game changer. I though it was the first decent one, even had a few laughs ("meh" was my default reaction to any attempt at humor in the previous eps), so I decided to at least try and finish the season.
The next three offered a definitive upward momentum in terms of storytelling and humor. There was literally no dialogue, no sentence without at least a dash of perfectly dosed humor.
I watched the season finale alone, and it was so funny at moments it made me spew random comments and laugh out loud all by myself.
It is a great show, it is a unique show - please, PLEASE don't give up on it! Watch till the end, you won't regret your choice.