Lemon Curry.
My book purchases are very much based on the contents of the particular book.
Thus far, I've picked up all 6 "core rulebooks" -- PHB I and II, DMG I and II and MM I and II. Assuming that future books are of about the same level of quality and contain similar contents, I probably won't pick up future PHB's -- all the content I'd be interested in will be in the Compendium. If future DMG's are as solid as the first two, I'll get them for the "how to be a better DM" content. Future MM's are a toss up. On the one hand, I'm going to get all my monster stats from the Compendium, so I won't need MM III+. On the other hand, I like monster books, so I'm inclined to get them whether I need 'em or not.