I only watch stuff that's done. I hate neverending stuff like the millionth episode of One Piece or such shit. I don't like waiting on new episodes to arrive.
So when I watch something, I download it, and watch it. If I don't remember in my mind where I'm at exactly, I just open an episode and see if I recognize it. If I don't, I go back in the series, if I do, I go forward, until I'm at the episode that I haven't watched yet.
Most of the time, I just know where I'm at. If I don't, it takes like 30 sec max to find the place I'm in, less than the time it would take to enter my "watched" status on any service, and I only have to do that rarely.
I can't really think of any cons or why I would use anything else. All those services are just wasted time/effort to me.