A while back I took a car service from San Diego to LAX for a flight to Australia. Seemed to work out okay, though it costed my work a fair bit of money. These days, I just add the SAN-LAX segment to the flight if need be to avoid the hassle (especially since it's usually not much more on the fare), but that's not really an option for most. FWIW I believe I made the reservation through SuperShuttle but I don't remember if Execucar was handling those at the time.
Speaking of SuperShuttle/Execucar, though, my parents fly internationally a few times a year and take them from Riverside to LAX. They don't really like them all that much since they did mess up at least once and left them stranded at LAX without a ride back. The blue vans are much different than Execucar, however.