Generally speaking, I keep my cars for a very long time and I do modify them. I don't care about resale value.
Modifications I make are not cold air intakes either - on my Grand Marquis, for instance, I've got a larger MAF with a bigger air box, ported plenum with larger throttle body, flow matched injectors, underdrive pulleys, custom built 10" torque converter with carbon fiber clutches, Bilstein shocks with bigger Addco sway bars, etc. Then I've got other stuff done, like blacking out the chrome, body color grille, courtesy lights on the doors, auto dim rearview mirror, power adjustable pedals, upgraded air vents behind the dashboard to accomodate a 2-DIN headunit, sound system with amps/sub, etc...the list goes on. All of these have made it a better car in my book and customized to my taste, but this is one car I'm never going to sell.
The Envoy, on the other hand, is a daily driver but I got bored of certain things and decided to modify that as well. Got flow matched injectors, I'm ditching the clutch fan for a 4,200 cfm electric fan soon, getting a new PCM with a tune (old PCM will stay stock and put away), got an auto dim mirror (can never do without those) and I'll be upgrading to larger Eibach sway bars. I'm also going to replace the steering wheel/instrument cluster and clockspring to get the DIC in, upgrade the rear end to a 4.10, debadge/tint and get a pair of Isuzu Ascender taillights on the rear. Again, it's a matter of taste, but if I ever have to sell it, I'll revert back to stock.
Most of the time, the buyer hasn't a clue about cars in general and will never in their lifetime understand what you have done and what improvements came with it. I'll just save myself the headache, revert to stock and call it a day if I ever have to sell.