It was an alright series overall. It excelled in the area of Ash's rivals ( see Paul, and Barry), and faltered when it came to Dawn's rivals. Infernape had some pretty good development too. The battles in the series also improved quite a bit.
However, I felt the GF was kind of meh, considering it was seemigly the last time we'll see contest in the series. The last battle of Dawn, and Zoey was okay, but Zoey/Nando was far superior. Jessie got shafted pretty badly sadly.
The Sinnoh league, was also pretty disappointing in terms of a conclusion. Tobias was a terrible plot device, and you could tell the writers couldn't really think of a way for Ash to lose when he's at his best using all his reserves without legendaries.
The Paul battles, and the battle with Conway were very good though. The battle with Nando was the only one other than Tobias that I felt meh on. I felt that the Ash/Paul battle should have been in the finals, or at least the Semi's.