We are just using the 3.0 core books. I'm still running the Adventure Path series, and although we now have a grip on the rules, something comes up every session or two that is new to us. Our goup has a few of the splat books (Tome & Blood, Sword & Fist, Defenders of the Faith) but we haven't found a lot there to use.
I've never been too enamoured of all the crunchy bits that seem to be so important to others. I haven't used all the feats/spells/magic items/monsters in the core books, so I really don't need any extras, thank you. And with monsters able to take levels, I don't see much need of of specialized creatures (my players were shocked by a party of 10th level orcs because they were still expecting 1 HD monsters). To me (and our group), it is important to just sit down and play, without being encumbered by too many rules or options.