I currently own a w800i (european version of w810) and a d900. The d900 is amazing in terms of form factor and overall sexiness.
But the w800 blows it away when it comes to usability. The user interface is a lot better, and the battery life is a *huge difference*. The battery life on the d900 is terrible. Plain terrible. Especially if you use the music player and camera a fair amount, it barely lasts one day on a full charge. The w800 can go two days with moderate to heavy use of features, and much longer if you're primarily using it as a phone.
The SE user interface is my favorite (though I haven't really tried the new Nokia Symbian phones yet), and the music player, slideshow, and file management systems are much better than the Samsung equivalents, IMHO. I also prefer the SE calendar system. And the w810 will come with good headphones and an adapter so you can use whatever 'phones you want with it (or plug in into a home or car stereo using a standard 3.5mm cable). You can get a similar adapter for the d900, but it's extra time & money to order one. I use my w800 with Etymotic or UE 'phones and the quality is on par with my iPod. The included d900 stereo phones are okay, but nowhere near the same quality if you listen to a lot of music. And then there's the battery issue, again.
If you're considering a D900 and willing to spend that much, look into the new SE K790 and K800 phones. 3.2 mp cameras with a real xenon flash (not the LED "Flash light" on the w800/810 that honestly doesn't do much good), bigger screen with better resolution, same UI and music player features. Only issue is if you need the quadband. If so, you'll have to stick with the 810 or wait for the "US versions" of the new k790/800 to come out.
I personally don't like the Moto RAZR phones much. iTUnes on the v3i isn't bad, but I find it to be a bulky phone and the 1.3 mp camera lacks. The KRZR has caught my eye, though.
nk