I didn't get a new Nexus 5, nor do I have Android KitKat 4.4 on my Samsung Galaxy S3. However, others have managed to get the APK's for some new Google apps off the Nexus 5. I didn't venture to load the Google Experience Launcher - I use Nova Launcher Prime in favor of standard Samsung TouchWiz. But I did have to test drive the new Hangouts app - now with integrated SMS.
At this point, the Play store did not have the updated Hangouts v2 (only version 1.2 which I had loaded). I grabbed the APK, did the standard side-load procedure through ES File Explorer and tested it out.
At first startup, it notified me that it can handle SMS and asked if I wanted to enable it. Hell yes! And 'BAM', there it was - all my existing SMS threads showing up (including MMS messages) in the Hangouts interface. I tested with some texting to/from the phone and received messages in both the Hangouts app and the default messaging app - as I expected (they're pulling data from the same store).
I did not get the standard messaging app notifications when texts arrived - only the Hangouts notification in the notification bar - excellent! So no double notifications, but message counts is the only thing missing. As I mentioned, I use Nova Launcher - more specifically, TeslaUnread - to put the unread count overlay on things like GMail and messaging. It never worked for MMS messages, and now it doesn't work for SMS either. No unread count overlaying the Hangouts icon in my dock. Not to worry, with the notification bar working, I'm sure I won't miss any texts.
So even though no KitKat (bars or software) were received for Halloween, I still did get a treat! Now just waiting on Google Voice VoIP calling so I don't have to use Talkatone.