I had an original iPhone from launch, then after 4 years of dutiful service it died. At the time I was looking to cut costs, and didn't feel like dropping 200 dollars on a new phone, so I picked up a WP7 Samsung Focus.
It was a relative piece of crap, but for all its downsides, the OS was snappy, livetiles were nifty, and it just worked.
About two years ago I upgraded to a Lumia 920, once again mainly a cost consideration, this time in relation to a Galaxy Note 2. It's not perfect, but it's a damn good phone, even though newer models have been released.
The build quality of the Lumia lineup is superb. I've never used a case, and the phone has taken a beating - yet there are no major scratches on the screen and the body still looks and feels excellent. It has a "well made" feeling when you hold it in your hand, not plasticky and breakable like my old focus.
The OS has received a number of major revamps and remains lightweight and snappy. My older phone still runs it brilliantly. It's not like a 3 year old iPhone where the OS has slowed it down to a crawl. It's as fast or faster with WP8 as it was when I bought it.
Cortana is great. I love the reminder system, use it all the time and still probably haven't unlocked the full functionality of it.
As mentioned, the camera is unparalleled. My camera photos are incredibly good. The biggest complaint I have is with the front-facing camera, as it seems to be not sealed completely and I now have dust that obstructs the image, rendering it unusuable. Not that big a qualm for me, I never use it anyways. It was probably my fault anyways.
As far as the app selection goes, it is limited, but it is hardly what it used to be. Just about every major service has an app in the marketplace. Some are 3rd party, but it's pretty easy to distinguish which 3rd party apps are reputable and which ones aren't.
All in all I've had a very positive experience. Just about every major issue I've had with the phone or OS has been resolved as time has gone on, and as mentioned, the build quality is just superb. I really like it. I'm still going to consider jumping to a different OS in a few months, but it's hardly guaranteed I will. WP is definitely a strong contender if you're willing to give it a chance.