Like you, I really enjoyed the first four seasons. The first two seasons are probably my favorite. I had a hard time getting into season five but then eventually really enjoyed it. Season six was hard to get through. I liked season seven and thought the first few episodes of this season were fantastic but like you said, it kept adding silly subplots and at this point, I don't think one episode can make this season the thrilling season it should have been. It will have a lot of ground to cover though. The Masuka stuff does feel like a waste and I feel like Dr. Vogel could have been used in a more compelling way.
True Blood is another show that started off great but has kind of floundered now for years.