Honestly I thihk that rules reached their saturation point in D&D about a year and a half ago. There are some notable exceptions, like the DMG II, Weapons of Legacy, and (I know I'm going to get lynched for this), the climate books (Sandstorm, Strormwrack, and Frostburn). Other than the WotC stuff, which I always get in order to keep up since I design stuff, I've actually been much more excited about things that bring entirely new concepts to the game. Yeah, that's a broad statement, so let me expand upon that. In the last year or so, I've really only been excited about the following types of books: monster books, campaign setting material, custom builds, and adventures. Every once in a while a crunchy book catches my attention, like Chaositech, but those are few and far between these days.
I always like monster books, especially ones that create higher level challenges. Granted, the best monsters are in the original Monster Manual, but new monsters present cool concepts to be worked into the campaign and neat new challenges for the party. I probably own every monster book released for D&D and D20 by the major publishers, and a few from the small ones.
Campaign setting material is useful, at least from the worlds I use. I've used Forgotten Realms (on again and off again) since the original gray box was released. I have all the Eberron books because I intend to use it once I've completely wrapped my brain around it. I think Keith Baker and Jaymes Wyatt really did an exceptional job of bringing us a wondrous and diverse world with lots of opportunity for exploration.
I find the custom build books exciting just to see what others can do with the same basic set of rules. I'm talking games like Iron Heroes, which I really dig, True 20, and D20 Modern. These are cool because they give you the chance to take a detour and play the game differently. If I find one that my group likes better than D&D, we might even switch permanently, but so far that hasn't happened.
Finally, the usefulness of adventures is obvious. They save me lots of prep time. Every single one is useful. You can always scavenge encounters and stat blocks, even if you don't like the premise of the adventure itself. Or the reverse is true. You can keep nothing more than the fundamentals of the adventure and toss the rest, or parts of the rest.
While I may have had my fill of new rules, which is the main reason I haven't written any since I worked on D20 Apocalypse, I will always find a use for, and be excited about writing, the other types of material I mentioned here.
