California is a big place. What's the destination? The best route to San Francisco is not necessarily the best route to Los Angeles. Let us assume Los Angeles (Disneyland, Hollywood and half the population of the state).
I-40 would be my route of choice, April through October, if time matters, going to southern California. There is plenty to see and do. From Oklahoma City west, you are mostly on or near Route 66, and all the sightseeing places were developed in the 50s and 60s for that traffic. The sightseeing place we call the Grand Canyon is where it is because it is close to this route. The actual canyon is much more extensive, we just visit the place close to the highway.
East of OKC, Nashville, Memphis, and Little Rock are worth visits. To the west, Las Vegas, Death Valley, can be not too far out of the way.
In winter, for Interstate travel to southern California, I would try to get down to at least Atlanta, and take I-20/I-10.
But from West Virginia, you can also leave on I-64 to St Louis, stay on I-70 all the way to Utah, then down I-15 to Las Vegas and southern California. This route takes your through Missouri and Kansas (which I've learned to appreciate after 30 years on the Great Plains) then across the Rocky Mountains in Colorado, where they are more impressive, and onto the Colorado Plateau to see a half dozen of the best National Parks, Monuments or Recreation Areas in the Park Service inventory. Season for this route is late June through early October.
If the destination is northern California, I recommend I-80 if in a hurry, US-50 if not, but the season for getting across the Sierra Nevada is narrower. Winter can be very long in the High Sierra.
If the question is just about "what is the road like, I-40" then the answer is "it varies." Past few years I-40 has been totally closed for months, a couple times, through the Appalachians. The road is usually pretty good through Tennessee, Nashville is always a mess at the wrong time of day (2-3 hours twice a day) and West Memphis is always a congestion point. A small section between Memphis and Little Rock (crossing a flood plain) was notoriously bad for years, but they've just finished rebuilding it. Most from Little Rock to Oklahoma City has recently been rebuilt, but there may still be projects working in OKC, my preference is to use I-240 to bypass the downtown. West of the Oklahoma City suburbs (El Reno and beyond) is still in decent shape, as is the section in Texas to Amarillo.
Beyond Amarillo, I can't vouch personally. Last time I came through they were rebuilding the highway between Santa Rosa and ABQ, but that was almost 10 years ago, it has certainly been finished but is has been long enough that the road could be beat up again.
The standards for the Interstate Highway system produce roads for 8-10 year lifespan, less if states allow loads over 80,000 pounds, which most do. This means along any route some sections will be almost destroyed by traffic, some will be being rebuilt, some will have been recently rebuilt. Pick any of the highways, East-West particularly, travel 2000 miles on the highway, and you will encounter all three situations.
When time doesn't matter, I avoid the Interstate system as much as I can. From West Virginia I might follow U.S. 50 to Northern California, U.S. 60 towards Southern California. I've followed both from the Atlantic to the Sierra Nevada, both are interesting trips, both are quite slow trips, even when dropping onto the Interstate to get through major cities on freeways rather than surface streets.
Tom Test
Itasca Spirit 29B