I was wondering this too. With the iPhone 6+ coming out, I'm debating whether or not to get the Lumina 1520.
I just don't know now. I LOVE LOVE Windows 8+ (Cause I took the time to learn it) and I really think that phones powered by Windows could blow Apple out of the water if they just had people developing better apps.
Some apps are REALLY good but some are missing completely and some are done really poorly. That is where I think they take a hit.
Keeping my fingers crossed... Oh and trying to learn how to develop my own apps..LOL.
I would say that the integration as far as Passwords, Preferences, MUI IE and WiFi Networks work really well between Windows 8.x/RT and WP 8.1.x
But as Fran eludes to....it will get even better when there is a single Windows OS that runs across all ARM and x86/x64 devices with different SKUs, and then once that is also ported to Xbox...it will allow true Universal Apps in the Microsoft Ecosystem.
The integration could be better but like jnjroach said it's great for IE passwords and such. I really like the Bing apps that are available for both the phone and the SP3. Sports, news, weather ect. They are awesome!
If you have a phone app that's also available for the SP3, or vise versa, then the experience is that much better.
For instance I use Package Tracker that's available on both the phone and the SP3 and it keeps both devices in perfect sync.
Just traded my Note 3 for a Nokia 1520. Love the phone, disappointed with integration. For example Outlook doesn't sync unless you go through OneDrive. Hopefully Windows 9 will integrate a lot better. Get the phone, you won't regret it. The 8.1 UI and Cortana work really well. The home screen can now display background custom images which look cool behind the tiles. The tiles themselves are customizable and you can get all the info you need on the home screen at one time. There is rumour that a 1525 is coming soon but from what I've read its not a huge upgrade. It is amazing how fast the 1520 is given its age, the screen is gorgeous and the battery life is exceptional especially given the size of the screen.