I've had mine since Friday. Previous phone was a Samsung Focus (for the last 2 years), before that I had an HTC Tilt 2 (for about 6 months), and before that I had an HTC 8525 on Cingular (for at least 2 years).
1) The Lumia 920 feels and looks expensive (in a very good way). It feels solid and the weight to size ratio feels 'right'. When you pick it up it doesn't feel like a cheap toy, it feels like a significant piece of technology, something that'd you expect from a top-tier, modern smartphone.
2) The screen quality is beautiful. My Wife and I compared the Samsung Focus to the Lumia 920 side-by-side and there is no comparison, the Lumia 920 is crisp and clear screen, it looks wonderful. And I already thought the Samsung Focus looked wonderful.
I've been playing games on it all day, I'm liking it a lot.
3) The OS is very VERY smooth. I've experienced no lag, apps open up super quick and the LTE network is amazingly fast (testing it at home it's quite literally faster for me to use LTE than it is to use WiFi on my home 30mb/s internet connection). LTE gives me about 16mb/s while my home WiFi gives me about 10mb/s. Real world experience: The phone auto-sync'd my contacts and all Windows Live ID account info (like pictures, facebook stuff, email, etc) in under 1 minute on LTE. I was extremely impressed.
4) Tile customization. At first it seems kind of cool, but after a while it becomes an obsession. Setting all your tiles up the way you like is a lot of fun and really does make it your phone.
I could go on about the phone, but mostly it'd be about the WP8 OS itself. The most noticeable thing about the Lumia 920 is the screen, in my opinion. Videos/netflix looks awesome and it just works like it should.
With WP8 OS there seems to be a lot more configuration options than before. I like to tinker and it's been fun 'tuning' WP8 OS to my preferences.
My Wife is eager for her Yellow Lumia 920 to arrive in the mail, hopefully soon!