I own every product that isn't an adventure and some that are. Now that said, do I think the spell compendium would still be usefull? For me, the answer is no, but only because I have the majority of the spells printed already in a database I created. You can't get any better than that. But for someone who hasn't already taken the time to a project like that, the answer is YES! It's always great to not have to look through book after book for something that you might want to put to use. having everything in one easy to access location is an excelent idea.
Beside spells, my db is chalked full off classes, prestige classes, feats, creatures, NPCs, items, as well as the art from the books the stuff was found in, and much much more and from multiple companies and settings. All withe easy to use view and full text seaching. so you can see why the book at this time wouldn't appeal to me