we've lived in boise for 3 years (originally from southern california) and have driven to
disneyland twice since living here.
we've found that there are 2 routes that are acceptable (really safe, well maintained roads, lots of places to stop for food, gas, emergencies, etc.).
if you were to go through SLC and all the way down thru utah, to vegas, and then to disneyland, that's a good route. we did that route once (only we were coming from vegas to boise because we had visited family in kingman, arizona after our disneyland trip), and it was very safe, easy to navigate, with places to stop all along the way. there's an hour+ between vegas and the next town as you head to utah, but that's the longest gap without places to stop.
on our way down to anaheim both times, we left boise, drove east into oregon, caught the 95 south at burns junction, to the 80 west, and then took the 80 all the way to san jose, then south on the 101. we have family to visit off the 80 and off the 101, so after leaving my dad's in ventura, we take the 101 to 126, 126 to the 5. you can take the 5 directly to disneyland at that point. we veer off to the 405 to visit family in the south bay, and then we take the 91 to the 5 into disneyland.
personally, i would avoid the 5 between sacramento and valencia like THE PLAGUE, but not everyone feels the same. i found it to be an incredibly poorly maintained road, and the drivers were absolutely crazy. this is coming from somebody who commuted 4 hours per day from ojai to el segundo every weekday for a year, so i am no shrinking violet when it comes to cali traffic. i've grown up in l.a. traffic and navigate it very well. but the drive between valencia (six flags magic mountain) and sacramento on the 5, is one i will never do again in my life. it is very much worth it to me to add the hour to my trip to head over to the 101, as the 101 is a safe drive. plus, it's a lot prettier!
if you consider the route from boise west into oregon then south into nevada, be careful what time of year you go. several years ago we came up to boise from l.a. to visit my mom for thanksgiving, and we hit a snowstorm on the way back and had a pretty serious accident due to ice. it was very frightening, and that portion of highway between boise and winnemucca, nevada is desolate (the only really desolate section of that route), and there was nobody to help us. i wouldn't take that route if you are travelling during any time that might involve snow.
hope that helps!
edited to add: also, if you take the SLC route, things can get very harry around snowville on the 84, too. you this is just near the border of utah and idaho, and you have to drive over a grade. one of our trips from arizona to boise several years ago was very scary (heavy, heavy rain and lots of big trucks) in that section.
personally, i won't drive to visit family anymore during any months other than summer or very early fall, just because i don't want to travel in bad weather ever again (that one accident was very traumatic). but many people do every day, and it turns out fine. just wanted to mention that the section around snowville can be really bad weather-wise.
