The demo was great. The artwork, sound effects and general atmosphere are outstanding. I especially love the weapon effects; the wrench really feels and sounds like something heavy and powerful.
The amount of small detail in the levels is top notch. I spent a fair amount of time just examining the areas more closely before I moved on. This is also the first game I have played in over two years that actually seems to be well optimized and runs nicely. I was able to get a more or less constant 60fps, with a few minor exceptions, at 1024x768 with all the details turned up on my old X1900XTX.
That being said, it certainly doesn't strike me as the "OMG BEST GAME EVAR MADE!!1" that a lot of the review sites are saying. I'll definitely be buying this, but a number of aspects of the game leave something to be desired. In general, it has quite obviously been dumbed down for consoles. The levels are pretty linear and have minimal interactivity. I was hoping you could at least pick up objects. The graphics are rather inconsistent, with magnificent water and displacement mapping effects mixed with various low poly models and some textures that look like they're from the Quake 2 era. The gameplay is a little odd, as health is essentially infinite due to the respawning while ammo is difficult to come by (like SS2, but it doesn't really make sense). At least in the demo, I didn't see any evidence of the complex statistics and inventory systems that SS2 had, or the story detail and reading material of Deus Ex either.
Finally, it uses the Havok physics engine, and sure enough, I ran into a vibrating corpse at one point.