This question has been asked several times in the past so there should be many opinions given. I have air horns (train style) mounted under my truck with an onboard compressor. The compressor supplies 140 - 150 psi to a 3 gal tank.
I ran air lines to the horns obviously, but also to a set of 12v solinoids controlled with a dash mounted switches. This allows me to adjust the airbags while driving and helps to compensate for poor roads and is handy for hitching up and raising the rig to put the equalizer bars on.
Also ran an air line that terminates with a standard quick disconnect in the bed of the truck near the tailgate. The compressor will easily fill a 15" tire to 80 psi. My truck tiores (20" rims} taakes a l;ittle longer but well under 5 min if the tire is completly deflated. I use a HF mini regulator with an intergal guage. Attached to the regulator I mounted an tire valve fitting that locks on to the tire valve and works similar to a quick disconnect. I keep the regulator set to 80 psi so I don't need to fill, check the pressuire, rinse and repeat. I can hear the compressor run when I first connect the air line. I know when the tire has reached 80 psi because the compressor stops running.
The set up also works well for blowing out the air lines. Set the regulator to 40 psi, open low then point drains psi and and go inside, open faucets one by one. It's also handy for bike tires, rafts, beach balls,, etc. I used to use competer air (canned "air" used to clean keyboards and interior componets} to clean out the A/C filter. Now I use the air and a blow gun for that and for cleaning the truck bed after hualing wood to the campsite.
the on board compressor is also handy for using an air stapler or brad gun while doing reapairs. As you can tell by this lengthy thread that I find it a handy addition. Just my experience, your milage may vary.
2012 Laredo 303 TG
2010 F250 LT Super Cab, long bed, 4X4, 6.4 Turbo Diesel