I pick and choose titles based on what I want to run, not the desire to have the latest and greatest.
For D&D, aside from the 3.5 core books, I'll definitely buy all of the environmental books (
Frostburn, Sandstorm, Stormwrack, &c.) because I find them interesting and useful. I bought
Unearthed Arcana (and like it) but ignored the planar book, the psionics book, the dragon book, and all of the settings books (save
Silver Marches - I don't play in the Realms, but this is close to the kinds of campaign-settings I like to create). I picked up the undead book, mostly for use with my Modern game, after several months of thumbing through it and putting it back on the shelf.
I will buy the
Wilderlands of High Fantasy campaign setting when it comes out, as well as
City-State of the Invincible Overlord - the next D&D campaign I run will be set here, my first not using a homebrew world since...well, since the
first time I ran a
Wilderlands campaign in the late 70s!
For other d20, I will buy d20
Past and
Slavelords of Cydonia. I may purchase some additional
Mutants and Masterminds books, like the next
Annual when it's released in a few years (

). I'll be watching to see what other
Sidewinder: Recoiled stuff appears as well.
I buy one or two .pdfs a month - all Modern stuff.
So, I'm not done yet, but it looks like a book every other month or so - hardly a torrid pace.