I have the earlier model... xps gen 2 with a 6800 ultra. Here's what i can say about it...
pros:
1) Incredibly powerful - feels like a high end desktop
2) Awesome graphics
3) Dead Silent
4) The best laptop display i've ever seen.
5) Hell of a lot of connectivity options
Cons:
1) Heavy... its a backbreaker
2) Poor battery life compared to normal laptops
3) On battery, the gpu only runs at 300 core, 600 memory. On mains, its a lot faster at 450 core, 1100 memory. Shud be similar with the 7800GTX.
4) Incredibly expensive.