I've flown this leg a few times, both back in the '80's. One was LAX-SAN in a TWA L1011 (an almost empty flight), and the other was SAN-LAX in a UA turbo-prop! :shock:
Suffice to say that the guys are right - driving is much easier and I have done this many times since. The freeway is a good run and you will have no problems. Renting a car is pretty straightforward and the roads are well signposted.
If your friend is picking you up, traffic can get quite clogged up coming in to LAX, especially during late afternoons. Another idea to avoid this would be to take a Super Shuttle (blue minivans that you'll find on the kerbside) to a location outside of LA. For example, you could take a shuttle to Orange County, which is on the way to San Diego, and get picked up by your friend from there. South Coast Plaza shopping mall and the Westin South Coast Plaza are right on "the 5" at Costa Mesa - very easy for your friend to pick you up.