Well I've just finished playing through the whole demo and also did some multipreying.
Engine/graphicsdesign wise it's a step up from Quake 4. There's less of the annoying lighting problems people complained about with Doom 3 .. with many areas being very dark so you have to run around with the flashlight on all the time even though the base is built for humans - With Prey there is more light but I still prefer the lighting in other engines like the Unreal engine or the Source engine.
Prey seems to feature more vibrant colors and more detail than previous Doom 3 engine games - thats good.
Wacky gravity - Boy .. this can be confusing .. especially at first and in hectic deathmatches. I don't think the human mind is geared for navigating 3D spaces that has alternating gravity centers like Prey has. But as with everything else , we will adapt and overcome. Definatly takes some getting used to in multiprey. This goes for the other guys as well though .. I got lots of kills while standing on the ceiling shooting at guys on the floor that had no idea where I was.
It's less of a problem in singleplayer because you can set your own pace.
Weapons in prey are wacky and different , which I like , but they don't feel as versatile as the unreal tournament weapons to me.
The whole Native American spin seems misplaced to me .. more like an excuse to have super spiritpowers than as good storytelling.
It has a story however which is a big improvement from previous Doom 3 engine games, which were always very weak in that department. It's not a great story , but it's there.
The game runs smooth on my PC , and for a demo thats a very good sign. I don't remember Quake 4 or Doom 3 running this smooth.
To me it doesn't look like it's going to be a classic FPS .. one that sets new standards for gameplay or anything - But I think it will shape up to be a good and polished FPS that Human Head can be proud of. The weird gravity and extensive use of portal should at least be interesting to see even if you don't like the genre.
I predict some people will have to take breaks between playing because of this. Some people get motion-sickness from playing FPS games .. This game will likely introduce them to gravity-sickness too.