Reverse World is by far one of the most fascinating things about the whole Pokémon world; the design, the rules, its Ying and Yang position with the real world... truly awesome.
Giratina, one of the new legendary Pokémon who appear in this film, wasn't bad either, and getting a glimpse of Dialga, one of my favorites, didn't make the experience any worse (although there was a bit of a plot hole there in the end).
Shaymin, however, was one of the most annoying characters by far in the history of Pokémon. Perhaps intentionally so, who knows... Sure, there was comedy there, especially between Shaymin and Ash, but the Pokémon's behavior and attitude just didn't work.
Other than that, another movie with a good plot, but there were holes here and there. It didn't run smoothly. The beginning seemed a bit too slow and confusing, but after that it was a rather enjoyable ride to the end.
Newton Graceland didn't get nearly enough screen time or depth, or his relationship and life in Reverse World, and same has to be said about the "villain" Zero (who is one of the best villains in Pokémon so far, I think). They could have had a much bigger impact if we truly understood them and their history/motives.
Not a bad movie, and well worth the watch. Standard Pokémon quality for sure.