The only "must get" for me is WotC's Unearthed Arcana, especially after reading Andy Collins' comments about the book (on his message boards). Whether or not I like the book overall is irrelevant, strange to say.
The other ones I am seriously considering are as follows: Complete Book of Eldritch Might (I will have to take a close look at this one), Iron Kingdoms (both books), and Midnight supplements.
Anything else I purchase will be unplanned, somewhat akin to the "impulse" buy, i.e. I see it at a store and look it over there.