I absolutely loved the sections as Ray, as the funky slow-motion aiming thing worked really well. The sections as Billy were quite fun too, but a bit frustrating at points. It was a very original game, and I can't say I've played anything like it. I really enjoyed the story, and the graphics were pretty good so my computer hated it. I would recommend it to anybody, and if you've never tried it go and play the demo.
I also believe that it has DX10 features, so an added bonus there - I'm unable to try this out, due to being on XP and having a graphics card thats about as ancient as Stonehenge.