A lot of members here have adopted the 900 as a some kind of mythical wonder phone. Search these forums and you'll find numerous complaints about the other 2 members of the Lumia family. Nokia's dramatic fall-off in smartphone market share isn't all about Symbian, they've had plenty of hardware problems too. By all means check out this very pretty phone, but don't be shocked if you find that your Focus S stands up quite well to 'Gods own' Windows Phone.
I own a Lumia 710, and I liked the user experience of the phone quite a bit, I wouldn't consider it a hardware letdown at all. Sure, the Lumia 800 did register a number of key problems, and it was disappointing in that regard. However, the Lumia 710 (esp. at its price) is a solid performer, to the point where I've forgotten about my original Samsung Focus.
Focus had superb hardware (a flagship device in its day no less), but that isn't enough compared to the support Nokia is giving to the WP platform. Not only are we seeing good hardware in the Lumia-series, but also a consistently evolving user-experience. We got Nokia Drive and Maps pre-loaded to the device, then they added CNN, Transit, Creative Studio, Play To (albeit beta right now). They are poised to add more (e.g. Reading), and are going even beyond that by openly supporting more enterprise support on WP7 (through Good Enterprise) and finding their way on Rovio's press-releases over Angry Birds Space.
This is within the <3 months I had Lumia 710, in the 1+ year I had Focus all I got out of Samsung was that cubix app. I agree, Samsung Focus S has superb hardware and I am sure it is a quality device, probably higher quality than Lumia 900. However, compared to what Nokia is offering on top of the Lumia-series in terms of support to Windows Phone, there's hardly anything in Focus S that makes it special enough for people to ignore Nokia.