Read the book a while back, wasn't bad but I wasn't impressed. A splash of cool concepts tied to a meandering plot - not an unusual pit for authors to fall into.
In comparison, Bubba Ho-Tep was a better, tighter story - but then again, it didn't try to stretch its premise across a full length novel. From a scripting standpoint, its was well suited to feature film conversion, which IMO showed in the product. Didn't hurt that the cast was particularly talented, either.
I see potential for Coscarelli to have turned the source material for John Dies into something worth watching, but I can just as easily see the weaknesses of the source dragging down such an attempt. Where Ho-Tep for me was a "make a pilgramage to the nearest theater showing the limited release" event, it will probably take strong reviews to get me to see this one.