As a DM, I don't really use anything aside from the Core Rulebooks.
As a player, more than one game I'm in tend to open up the available options. Everything is under DM approval, but if I don't even know about it then I can't very well get approval.
Since I do not really purchase any of the extra books (don't use them for my games), the Spell compendium was exactly what I needed to play spell casters in the other games. Rather than buying every book for a handful of new spells, I have one book that doesn't duplicate the Core books (which I already paid for - five times for the PHB alone if you count the three 3.0 versions and the 3.5 ones for my wife and I) and give me just the few things I wanted out of the rest.
It is hands-down one of the most useful books I've seen released IMO. As a DM, I think the magic-item compendium will be almost as useful. I would not approve PCs using/referencing it in my games though - magic items should be mysterious and I tend to change small things about them all the time for flavor and to give a new twist to my 20+ year players.
Every now and again I think a Compendium for Feats would be nice.
As to magic items for lesser used slots (vest/shirt for example), I simply make my own... Shirt of Intellect +2! The enhancement bonuses don't stack, so no throwing on a headband of intellect with it. I've done that so long, I can't even remember if that's a real rule or my house rule.
