Played the demo, was impressed with some of the features, it had a lot of little 'cool' things, both in terms of game mechanics, and in details in the levels/weapons/hostiles, but it didn't once scream 'buy me' in any way that would even remotely make me consider if over Half-life2.
I'm not big into FPSs for their own sake, though I can very much appresiate them as 'action games', which is somthing that HL2 delivered very nicely. But Prey, like Sins, is probably great for someone begging for a new FPS fix, it's cirtainly very compitant, but it's still really just for the FPS fans.
EDIT: YES I KNOW THAT SAYING THAT AN FPS IS FOR FPS FANS IS MILDLY STUPID, BUT YOU KNOW WHAT I MEAN. RAGE.