I use a 128GB original Surface Pro for my main computer (we are on a 1 year assignment in the States) including Photoshop, Lightroom, etc. I would be happy to answer your questions.
It is plenty fast enough. I have my processor clocked way down when I'm on battery and only then does it feel slow. My other computer back at my flat in China is a Core i7 overclocked at 4GHz with my Lightroom db on its own Raptor and a boot SSD. So it's not quite as fast as that. I would say, overall, probably 80% as fast and that's with my LR DB stored on the microSD card (much slower than built-in SSD).
Really the only thing you have to work around is having only 128GB SSD onboard, but you will have that in many other laptops as well. I believe the Surface Pro 2 is available with more storage, but again as with other laptops, the price climbs quickly.
The portability and the versatility of the Surface Pro are what I love about it the most. It can be a tablet, a laptop, or a mobile workstation (I carry a small mechanical keyboard and a bluetooth mouse sometimes). The screen is absolutely gorgeous, and the pen input is great when you need it (~1000 pressure levels, even in Adobe apps). Just don't ever try to use Lightroom or Photoshop with touch; the buttons are way too small in those apps.
