Tons of issues fixed for me in b3 (on an unlocked VZW iPhone 5):
-Can now create new messages without Messages crashing
-Can now dictate messages via the keyboard without Messages crashing
-Safari is a lot faster
-I can answer phone calls 100% of the time without freezes/lag
-The lock screen isn't intermittent (e.g., I don't have to mash power 10x to get it to show up)
-In Messages, keyboard clicks are no longer randomly toggled on/off
-In Messages, autocorrect seems to work most of the time, as does predictive text
-Photos doesn't seem to be crashing randomly anymore
-Camera app is a little snappier
One issue I found was when I tap a link inside of a Facebook message, the app crashes. Very minor annoyance considering the vast array of issues that I've seen fixed.
My favorite: although my late 2011 Macbook Pro HRAG does not have low-energy bluetooth (yes I checked), when I loaded up Yosemite b3 I was able to take a phone call from on my computer. I felt pretty cool.

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Any idea on the public beta of Yosemite ? Has anyone got it ?
Seems pretty stable to me. I obviously haven't used it extensively, but aside from a few Quicklook crashes when I wasn't even using it and stubborn Mail messages not being deleted the first time, apps I use often like Transmission, Photoshop CC 2014, Safari, Mail, The Unarchiver, Little Snitch, Beamer, Sonos, and Reeder seem to work just fine.