The Forgotten Realms products I use the most are the FRCS (of course), Magic of Faerun, and Monsters of Faerun. I also use Lords of Darkness for reference when I'm designing enemies for the PCs to face. I also suspect I'll be getting a lot of use out of The Silver Marches book soon, since the PCs in my campaign are approaching that area.
Frankly, I love the Forgotten Realms setting. I can't think of any fantasy setting that is deeper and more richly-developed than Faerun. Besides all the current suppliments for FR, there are also scores of 1e and 2e FR suppliments that still contain valid information that players can use in their campaigns, as well as a host of older-edition modules and adventures that enterprising players could update to 3rd Edition. And if these weren't enough, there are more FR novels out there than you can shake a stick at. Granted, most of the novels are hit-and-miss, but if you can find a good one to read, it will give you a good "feel" for the realms.
Also, the FRCS is how a campaign-setting book should be done; Over 300 pages, hardcover, beautiful illustrations, new feats and spells, info on geography, deities, and NPCs. Oh, and like Henry mentioned in his post, the book is pratically overflowing with plot hooks and adventure ideas. Ever since I got the FRCS, I don't think I've picked up my Living Greyhawk Gazeteer once.