As a player, I frequently consult it when performing some unusual maneuver or heading into strange territory (underwater for instance). Aside from my notes, this is the only book I usually like to keep on the table for reference during the game, and then, only to resolve important rules disputes (such as the death of a character). The Rules Compendium is handy to keep around. There is lots of helpful information on making up quick NPCs that I feel is superior to the NPC tables in the DMG. The Player's Handbook II I find to be a most valuable resource of character building tools and variants.
Whether you are looking to tweak your game just a little bit or drastically alter the mechanics, this books offers a trove of ideas for simplifying, expanding, and just snazzing up your game. Unearthed Arcana is, for me, indispensable to running any 3.5 game.
