Absolutely amazing. This was Pokemon's Sozin's Comet, and it was freaking epic.
Between the two episodes though, the second one was good, but the first one was really really really really good. Everything about it was awesome. In general, seeing everyone attack, having the XY theme song play with the gym leaders, giving everyone a major role without overdoing it, Serena actually having a decent role and some time in the battle, Team Rocket helping then being saved by Diantha, Blaziken Mask reveal, Diantha joining the fray and joining Steven as a champion duo, Lysandre, the aerial battle, and probably some other stuff I'm forgetting, everything about both was awesome. Though I do think the final fight between the megalith and Perfect Zygarde was over too quickly, overall the two-parter was absolutely stunning.
Though the way Bonnie essentially cheered for Perfect Zygarde to destroy Lysandre...that's pretty dark.
Overall hands-down amazing two-part finale. The first part is currently contending with XYZ028 as my favorite Pokemon episode of all time, and the two-parter managed to not only give awesome action but managed to bring together many of the main characters we've seen throughout the series without overusing any of them. Like I said, this finale was Pokemon's Sozin's Comet, and considering Sozin's Comet was my favorite Avatar episodes, this is no small compliment.