When I DMed I never had any interest in the splatbooks; only the core 3 books and the MMs. Ironically, I think, though I knew certain parts of the rules very very well, I knew almost nothing about feats and PrCs.
Once I started playing regularly (I have played for the longest continous period ever in last year) I LOVED the splatbooks for designing feat combinations and for making characters with just the right mechanical abilities to match the character background.
I guess this what I love about 3.5; there is a chance to actually create almost any character and have the mechanics back it up so that it is not just a bunch of fluff that the DM can ignore. I haven't ever seen an RPG that is so complete in this sense, although the big problem is players who won't invest the time getting left behind. Having said this, I have designed quite a few characters as backups (I used to keep getting killed, I think because I was such a pain in early sessions and because other players often would mess up tactics) and these replacements have ended up becoming the PCs for other players and hence I designed half of the party.
Going back to the "are you a problem player" thread; I am, and so I needed something else to engage my brain to stop me being such a pain.