Finished this last night. Took about 12 hours to complete, and I didn't rush through.
The ending was great!
I don't think it's necessarily a revolutionary VR game in pioneering new techniques and interactions. It has however taken what a lot of other games have done before and polished or enhanced them. Lots of VR games use a 'grabbity gloves' style mechanic - it's sensible because you don't necessarily have room scale, and crouching to pick things up can be tiring (or even not possible for some people), but they way Valve have done it here feels so good, with that flick of the wrist and the following catch.
The hacking puzzles were probably the best interactions in the game, as they really required 3D stereo and hand presence to play them, plus the sense of presence when manipulating them was very real. It was easy to forget you're in VR while solving them.
Graphics and sound are some of the best I've seen in VR. I think Lone Echo comes close in graphics, polish, and presence, but of course that's a much more limited and shorter game.
The main criticisms I have are no melee, as you really want to whack those headcrabs, but can't. (Apparently they tried a crowbar, but players kept on getting it caught on things as they walked around), and the small, linear levels with little side exploring. (I know, HL1 & 2 were linear, but even they had large levels with side areas to explore).
If this wasn't a HL game it would still be a darn fine VR game, but the added HL and the lore and references take it up several notches.
The killer app for a system isn't necessarily the most revolutionary (Boneworks), or the most accessible (Beat Saber), but its the one which can generate the most hype and publicity, and cut through to the wider gaming public. I think this is the one for VR.