I already had Delta Green and loved it and Countdown just expanded and gave more depth to the whole theme.
This is a big book and although I usually love having the actual book in my hand having this in PDF is very very useful.
If you have Delta Green and like it, you will love this
Here is a quick video review I did for Delta Green Countdown. For those of you concerned as to the quality of the printing and the binding of the Print on Demand version on sale here- http://youtu.be/J050iHSeGxI