I think that they do the job of giving the core game the feats it needs. After that, the feats aren't really essential, but they allow a lot of customisation of your characters - more choices, more variety. That, of course, is the point of feats.
I'd say generally yes, the core books are more then sufficent to play good D&D. But if you have access to other books they can add to the experience.
The divine type feats from Complete Warrior should be a good addition to any campaign.
You know, I think my group makes so few characters in my campaigns the players have not fully explored the possibilities of the core rule's feats. I could see where folks who have played a lot of different character concepts might want some more options - we may be there in a few more years
For me, they are generally enough. Even if I have rolled-up a good quantity of characters lately, the way to make different one from the other has more to do with the way I play them than with the feats (or other mechanic option) I give them.