Psionics Handbook is a big use. I play alot of psionics in my game and have a psychic warrior. I also frequently use the splatbooks for all the classes, especially the Tomb of Blood and Sword and fist. Two PC/NPC prestige classes. I use many Prestige Classes for my npc's to throw my players off and take them by surprise. I don't play a FRCS, but use the book for ideas. BoVD has been used once for a particular group of characters and it was very useful, but other than that never used. Not Epic yet, but when and if it does happen will be frequently used. OA was used minimally, but I do have an orientlal area on my world map which is yet to be explored by the players. When they do it will come in mighty handily.
To be honest all the books are great to have when the need arises. You may not use each of them all the time, but when you are looking for that odd spell, prestige class or idea they are all invaluable.