I just finished playing it for about 20 mins. The gameplay of COD feels really familiar despite the modern setting. If you're a COD fan, then you're going to love this. If you didn't care for COD, you still won't like it now despite the new setting. I liked what I've seen so far but I wish that the SP mode were as innovative as the MP mode.
Technically, the developer has done an amazing job. Despite having many upgraded effects over COD2 (COD3 wasn't on the PC), COD4 runs much better than COD2. I get 60fps 90% of the time with maxed out settings at 1920x1200 with 2x AA. With AA off, I don't think it ever dropped below 60.
It downloaded really fast from filefront so get it there if you want to give it a try.
http://files.filefront.com/Call+of+Duty+4+Modern+Warfare+Demo/;8774322;/fileinfo.html