I'm on record as being in favor of the large Marvel Omniboo.  Marvel has really worked out the early difficulties associated with manufacturing such thick volumes.  The Thormnibus and Byrne FF volumes are great, and I'm counting down to the X-Force/X-Statix Omni!
 
That said, there is a physical limit as to the size and weight of a single book block beyond which no binding can really hold up.  With that in mind, I have to wonder if DC will be using a lighter weight paperstock for a book with this many pages.  No matter how much I might be interested in this book, I wouldn't purchase it until the reviews came in.  I wouldn't want to take a chance on purchasing it and then discovering that the binding is inferior or unable to support itself etc. 



Reading now:  White Tiger mini-series; X-Statix Omni
 
Awesome new acquisition:  X-Statix Omnibus (still waiting for my copy of Marvel Firsts 1960's to arrive, I'm sure that will qualify)
 
...with great power comes -- great responsibility!