I upgraded from a Nokia N95 and couldn't be happier. Feature-wise, the two phones are almost the same, but the performance of the X6 is vastly improved. Sure it's not hard to beat an old phone, but this one leaves me wanting for nothing.
Like all top-end Nokia phones, it has great built-in features: GPS, camera/video, music player, FM radio, wifi, 3g.
Apps load very fast, and the native browser loads really quick too. Wifi reception is great, and the GPS locks on within seconds.
The glass touchscreen is very accurate and easy to use; the display is beautiful. The haptic feedback is a little inconsistent or delayed, but it doesn't distract.
Menu layout is improved too, and once you figure it out, everything is within a tap or two. The Contacts layout on the home screen is hugely useful.
My one complaint is headphone volume is too loud or soft; the controls are not fine enough. That was true on the N95 too.