The I-5 is so boring though, at least on 101 you get a scenic drive and it curves quite nicely in places; I-5 is all straight. If someone has not done SF to LA before, I'd definitely recommend the 101 and/or PCH as much as possible, even though it adds a lot of drive time to the trip.
Also the 101 merges into the 5 in Downtown, LA and from there to San Diego the 5 is the only route, unless you take 405 which also merges into the 5 later.
You can always take 101 to PCH through Ventura, into Malibu, and then Santa Monica where you can pick up the 10 to 405 and head south to San Diego. Much nicer than driving through Downtown, LA.