This update is the best thing to happen to Minecraft since Survival Mode. Not only does it double the world's height, but it also adds new mobs and a new (and awesome) biome. Jeb said that it would slightly raise people's performance on their computers as well due to all the improvements to the biome generation.
Even though I really like Notch, I gotta say that replacing himself with Jeb and Jon was the best thing he did for Minecraft! Notch put amazing things into the game, but seemed like he was losing interest throughout 1.8 and 1.9, and just ignored the community. Jeb is listening to the community and the game is improving so much, or at least this is a good start. I'm glad Jeb has someone to help him out so he could could do this faster but staying at steady speed. You know the saying: "2 heads are better than 1!"
Fantastic. It is so nice to see some more content updates frequently now that MC has made its official 'release'. Either that or Jeb works more efficiently than Notch did. Either way, good work you crazy cats!