Diana Riggs' scenes are entertaining and enlightening, they give us more of a sense of who the Tyrrells are, and why the Tyrells matter in the Lannister-Tyrrell alliance (and that'll be very important once that starts to sour next season). They serve more of a plot purpose than say, the endless faffing about Arya did in A Clash of Kings.
Arya has the best storyline in A Clash of Kings
! The almost post-apocalyptic landscape she has to journey through is harrowing, and the atmosphere in Harrenhal was thickly atmospheric.
The Tywin and Olenna scenes pointless beef up King's Landing's screentime, when it is the place where the least amount of stuff is happening right now. Screentime that could have been given to maybe setting up Mance Rayder or Roose Bolton properly.