most of what other people have said i would echo. adventures are by far the thing i am most likely to buy (and, to date, the only thing I have bought). Unfortunately for 3p, aside from any grumblings about OGL vs GSL, WotC is too good at producing material for their own base, and their print quality compared to 3pp is impossible to approach.
The poster who mentioned manually importing 3pp monsters into the monster builder has convinced me to reconsider my stance on monster books. Honestly, I would go a step farther. In the latest WWDDGD, part of the event was building your own encounters. to do this, they provided sheets of monster stats, clearly constructed from the stat cards that come in DDM packs, that you could cut apart in order to group cards together by encounter. This is a brilliant idea, and layout is probably the biggest hurdle to integrating 3p monsters into a DM's personal prep. Give me a "Character Power Cards" sized box full of monster stat cards, and possibly a handy reference booklet for all the non-stat stuff (history DCs, sample encounters, tactics) and you have a strong product, imho.
