I had previously watched well over four episodes, from seasons 1-3, and been mildly entertained, but not hooked. When those seasons were rerun the previous time I somehow got to watch several consecutive ones and then wanted to see more and looked them up, and shortly after that I was addicted.
A lot of it has to do with the charm of the individual doctors, 10 and 11 do a pretty good job, (9 may have, but I think we just got too little of him to get attached - it took me a full season and a half to have 11 grow on me).
But Strax is a delightful combination of the writing and the actor together really pulling it off to make a rather minor character memorable.
I am pretty sure he will be in episode 9 of this season as well, looking forward to that.