I've run the gamut of smartphones over the last few years (iPhone 3GS -> HTC Inspire -> Lumia 900). Of the three, I've kept the Lumia the longest (over a year now) and given that I still have it, I'm pretty happy with it (it's true, I have historically gotten new phones almost every year).
There are lots of things I like about the Lumia - the overall design, size and feel of the phone is very nice. I haven't had any issues with broken charging ports, etc, and let's just say I'm not always 'delicate' with my tech. All the buttons respond appropriately and the screen responds to touch the way it did when I took it out of the package.
Windows 7.8 (even 7) is intuitive, stable and moderately pleasing on the eyes. The ability to customize the layout definitely surpasses the iPhone and even the HTC. In fact, when I recently purchased a new laptop with Windows 8, my contacts ported in automatically when I synced up my Windows Live email address from my phone (which I wasn't even anticipating).
Call quality is usually pretty good (any problems I have are usually network related, and usually it's not the fault of the providers). Even the voice recognition works pretty well.
Obviously, it's not the end-all, be-all. There are shortcomings - it's last generation's technology (predecessor to the Lumia 920), the phone cannot upgrade to full Windows 8, no native ability to capture a screenshot, and the Windows apps are nowhere near catching up to iOS/Android, which was something I hoped that would at least improve in. Finally, while I'm biased in that I own lots of expensive photography equipment, I'm not really all that impressed by the camera on this phone (it works, but I would rather go through the trouble of carrying a decent point and shoot and import to my iPad than use the camera on my phone). I guess that's not really a dig on the phone itself though, as I felt pretty much the same way about the camera on the HTC Inspire.
All in all, for an unlocked smartphone, I think this is still a pretty good value if you're looking for a reliable smartphone without a contract.