This tale introduces Rantaplan (or Rin Tin Can), and with him greatly anthropomorphizes all animals in general. Even Jolly Jumper speaks now. It adds another layer to the stories, a fresh new canvas to fill up with jokes, so I have few complaints. I never did like the dog as much as many others, though.
As for the Daltons, I feel that they're getting a little old now. Jack and William have lost every unique characteristic they had and are just becoming the bland middle brothers, whilst Averell continues to get dumber (and hungrier), and Joe is consumed by his hatred for Lucky Luke. They also dress up always in their prison uniforms, which never worked for me too well either.
Still, the action and comedy remain intact.