I haven't used it myself, still on the ol' K800 (I say 'old' but you know how fast mobile technology advances!)
From what I've heard from everyone, and the reviews it looks bloody good. Great new media interface, HSPDA, great camera (As usual for the K series) and the usual SE OS. Plus Mini SD support as well. Another cool feature is the battery, sim and memory card access panel is accessed through a digital camera-like hatch.
If the iPhone project burst into flames, I would certainly buy one of these.
On the negative side however, a friend of mine says the keys are pretty bad for texting with and the touch sensitive keys take a bit of getting used to. Check out mobile burn site for a good review!