It gets progressively better.
With each series, you come to identify more and more with the characters and their various roles and sub plots within the series.
With it becoming a global hit, they throw more money at it, which means better sets, better CGI and better battle scenes.
There`s not one single actor in the whole series, who is miscast and if anything the introduction of new characters in each new series, without killing old ones off, just strengthens the series, rather than doing the opposite.
If I`m going to be hyper critical, the final series is the weakest, but that`s only because they had to find a way to wrap it up.
People who slag it off, either have never watched it or have no imagination, it`s as simple as that.