Fallout: New California adds an all new story around a new player character, an adopted resident of Vault 18, embarking on a journey through the wastelands of the New California Republic's Cajon Pass. An unofficial prequel to New Vegas, FPB adds hours of new gameplay and a fully voiced stand alone campaign.
This mod completely changes the way the game is played. It's basically getting an entirely new game for free. With emphasis placed so highly on the story and characters this mod hinges on those performing well, and they do for the most part. There are a few places where the dialogue is a bit iffy, but to me that's all the charm of playing a mod with voice acting - it reminds you that this was made by people who love the game just as much as you do. I really can't wait for the full release of this mod and look forward to it immensely.