This was my 2nd game that I downloaded from Steam yesterday, I booted it up with no problems, but then once I got inside the game world there was really messed up shadows. 
 
I went on the forums, and there were some very helpful people who provided some solutions. I tuned all of the lighting effects OFF, and made sure netbook mode was off, and render behind player was off. 
 
But yeah, it runs well on my imac with 2.0ghz intel core 2 duo, 3gb of ram, and only 128mb of vram from my amd radeon hd2400.

Surely running well would be with lighting effect turned on, no?

MIne's running perfectly, most of the settings are on high, i got shadows and it runs seamlessly. i got a 2.4 Ghz Macbook white 4 gigs of ram and the integrated graphics card from intel

Mine runs great, everything turned to max. The load times between levels are a tad long, but other than that, it's super smooth. I'm loving this game!