There are two sides to the Lumia 800.

The hardware:
Looks great, amazing build quality and is one of the best looking phones that has come out for a while. It has the usual good Nokia optics (although I found the quality of the pictures a bit hit and miss) and uses a better flash than on most phones.

The screen is clear and the text is easily legible, although I found the browser to be a bit pixelated when zoomed in.

Its only single core but you wouldn't notice.

The Software:
I love the metro UI style and WP7 is a fresh approach to a mobile OS. The issue I had was the availability of apps that I use to get through my day. There isn't a massive amount of apps at the moment and although you have the basics such as a Facebook and Twitter client there isn't a lot of apps that I rely on such as productivity e.g. a good to do one.

Give it a year or so and I think WP7 could be in with a chance.

So hardware = amazing
Software = great UI and got promise but needs more apps.