We rode from San Diego to Reno using the SC except at the ends of our route and were not on I5, but yes it does use I5 for a bit near the CA OR border.

I have ridden in the interstate before and found the experience varied from just OK to very nice in the cases I have done it. On the Trans America we were on I80 for a short ways and it was safe enough, but not all that aesthetically pleasing. On my 2009 Santa Fe Trail tour I rode from Springer NM to Santa Fe and much of the day was on I25. I25 in NM was actually very nice. Clean wide shoulders, good scenery, traffic not all that noisy most of the time.

I do not know how I5 is, but I wouldn't let the fact that it is an Interstate worry me much.