I try to build OOB as much as possible but inevitably add some AM details.  For me it really depends on the quality of the kit and how realistic I want it to be.  For instance, with the B-17 I'm currently building for the contest I have gone hog wild with the details since I want to accurately depict a B-17 so I added AM gun barrels, gunsights, ammo belts, flaps, wheels, and control surfaces.  If I wasn't doing a diorama or wasn't following an photos of the specific plane and just wanted a display model I most likely wouldnt have added all those details.  On the contrary, all I' added to the P-47 I'm building is a resin seat because the kit came without seatbelts and I plan on having the canopy open (of course it doesn't hurt that this is one of Tamiya's best kits).  With the Revell F/A-18E Super Hornet I plan to do this summer all I'm adding is a better detailed resin ejection seat, despite reading an extensive review that pointed many "inaccuracies" of the kit.  Then again I'm not as passionate about jets as I am WWII prop planes and that passion factor also depends on how detailed I want something to be. 

That said, I try to keep AM at a minimum and will never spend more money on details than I spent on the kit itself like some modelers do.  I try to walk that thin line between details and a resonable budget and so far I've been doing pretty well