I really didn't care for the resulting battle between Pikachu and Serperior; I feel that in trying to go a different route by making Ash battle his main rival first for a change, the writing team messed up by making the resulting battle rather poor. The whole thing just felt a little unbelievable, which is a shame.
I don't know if they were trying to make up for justifying Paul being a git in Sinnoh, or what. Trip was an interesting enough rival who deserved a better send-off than this.
There were also some misses in Bianca's battle, but at least it was more satisfying as a whole.