Only time I might consider a detailing is if I'm trying to sell the car. Otherwise, I think it's overkill.
I used to be pretty bad about washing my car (maybe 2-3 times a year if that?) but I realized that some of the pollen/tree sap damages the clear coat, so now I try to do it about once every other month. I go to the self-wash place and spend about $5 to scrub all the gunk off myself, and consider it an interesting form of exercise. I also get free auto car washes with my oil change about 3 times a year (my mechanic made a deal with the car wash place right next door, so win/win for them) and use their vacs (also free) to get the grass and bits of grunge out of the floorboards then. I don't spring for waxing or any of the fancy stuff, just a scrub off the build up of road dirt and tree sap/pollen.
Oil changes are definitely good to keep up with, but don't forget tire rotation, checking the air pressure to make sure your tires are properly inflated, and checking fluids (about once a month). Change your air filter regularly (most of them are REALLY easy to do, but my husband's car has a weird setup that gives me fits... and yes, I'm the one doing his air filter changes).