Played through the Borealis Prologue last night. I think they've done pretty well capturing the feel of the HL2 series, the efforts they've gone to with the menu system etc are noted. However, it all feels a little bit "uncanny valley" as it's a bit off. Minor complaint though.
The world is very good, it feels like another Source technology iteration like we had between HL2 -> EP1 -> EP2 and as such it feels natural. Gun play is pretty good, I think the pistol is spot on but the shotgun felt off. It was either the rate of fire, or the reload but it didn't feel right.
All in all, I'm excited to see where it goes. Shows a lot of promise, and a lot of effort has gone in to realising the potential of the story.