I finally got around to playing Geist last week. I enjoyed it for the most part. The puzzles, while limited, were fun.
In my opinion, the weakest part of the game was the weapons. Every gun felt the same, whether it was a machine gun, a pistol a rivet gun or even an demon blue-fireball thing. They were all very underwhelming. They needed more kick, better sound effects and a different feel for each.
I also didn't like the ammo system (i.e. unlimited ammo sucks). I would have much preferred having a limited number of clips available for any one host. My favorite part of the game was the second boss fight against Cord (the one in the room with the other soldiers and the two gun turrets). It was pretty frantic switching between hosts in order to kill him. There needed to be more fights like that (not necessarily boss fights). I would have liked to have stepped into the middle of a huge fight between the demons and the humans and have to possess a host, kill until I run out of ammo and then change hosts; wash, rinse repeat.
Also, the game went downhill a lot once you regain your body. I can think of only one time during that entire part where I had to leave my host. Sure, the time-warping toolwas kind of cool, but not nearly as cool as floating around in specter form.
Other than that, it was fun. As most people have pointed out, the game was too linear. But that could easily be fixed in a sequel.