Is it still a big deal like back in the ‘good ol’ days’ (last gas for 200 miles, etc) or is it pretty mundane now?
Also, what time of year is best to cross it?
(I used to live in the extreme western tip of the Mojave Desert.)
People cross the desert all the time. It’s how you get from L.A. to Las Vegas. There are plenty of places for fuel and food. There’s not a lot east of Barstow, but there’s still civilisation. For example, Baker is about halfway from Barstow to Las Vegas.
Any time is a good time to cross the Mojave Desert – though in Summer you’ll want air conditioning. In Winter you’ll want a functional heater. Be sure to take in the beauty. Springtime brings flowers and other vegetation. I like the variations in colour among the sand and different rocks. The mountains are beautiful, especially if there are some white, white clouds in the blue, blue sky.
FWIW, the original road – as in 1800’s riding a wagon road – from Barstow to Las Vegas is still there. I’ve never been on it, but I’ve read that you’ll need 4WD to make the trip.
About how cold does it get? I live in South Dakota so a functional heater is a must even though last winter wasn’t all that cold and it sounds like this one will be the same.
I do it all the time. You can drive through the Mojave desert in a day, and if you stay on a major highway, there is plenty of food and gas to get you through. No big deal.
Not that cold! 
I lived in Lancaster, and it almost never got down into the single single digits at night. I used to drive by a bank with a temperature display, and in the pre-dawn hours in late-January it was usually in the 20s and 30s. I don’t know how cold it gets around Barstow, but it has snowed there.
For Summer trips, do bring plenty of water. There’s a fairly long grade going up into Nevada, and cars often overheat if their cooling systems are not up to snuff. (Turning down/off the a/c is necessary in older cars.) If you get stranded, try to park under an overpass and stay hydrated.
Yeah, it’s not a long trip. It’s less than 300 miles from West L.A. to Las Vegas.
As you say, no big deal.
I just got back from Wendover NV, we had 165 mile stretch with no services. That is about the longest one I can think of if you stay on main roads.
So about 4.5 gallons of gas. 
Car should be OK as it’s a 2012 Ford Fiesta I’ve only had for 6 months.
Thanks for the replies so far!! Much appreciated.
I live in the Mojave currently and like everyone is saying most of the time it isn’t a big deal but this is very good advice. I’ve seen plenty of cars break down in hilly areas. It’s still getting to about 110F here so be nice to your car 
We live in a less populated area and every so often end up rescuing someone stranded out here. There are some places where there is no cell reception and if you break down you’re on your own. But if you’re just crossing you probably won’t end up in one of those places.
Dehydration can be a little insidious. Being a dry environment (which most of Southern California is), the sweat tends to evaporate quickly. If you’re travelling in the desert during the Summer months, drink frequently even if you don’t feel thirsty.
How can you tell the newcomers to the desert?
They’re the ones with the tans.
(People who live there know to stay inside where it’s cool! 
)
Hottest temp I’ve ever experienced was gassing up the car in Baker, middle of an August afternoon. “The World’s Biggest Thermometer” read 124°.
The wife of one of my old bosses died out there of exposure, after breaking down. No idea where or how it happened.
I found out that a woman I worked with for four years at Edwards AFB died of the heat while rock hunting at Calico Gold Mine.
From where to where around Wendover did you go for 165 miles with no services?
(Which road?)
I went straight up highway 93 out of vegas. Might have been 130 miles?
I went through it in December of 2006. It was colder than I expected due to the high winds. I had no trouble finding food, fuel or anything else I needed. Other than the scenery I didn’t think it was any different than driving from Dallas to St. Louis. That said, I packed a little extra water just in case.
I’m guessing the 100+ miles of no gas section was actually Nevada 318. Between Alamo and Ely, Highway 93 scoots east to hit the metropoli of Caliente and Pioche (in which one can purchase gas), whereas 318 is a slightly shorter route that blasts through about 150 miles of absolute nothing in the next valley over.
As to the OP, one of the big problems with traveling in the more remote Mojave highways was that the little out-of-the way gas stations would basically maintain banker hours. So if you rolled up at 9pm with the needle on empty, you were probably spending the night. These days 24 hour pay at the pump is mostly (though not entirely) universal.
It is a good idea to carry a few liters of water in the car anyway, all the time. Crossing the desert, even on a freeway, double that. If you go off the freeway, bring several gallons per person.
Desert-crossing is best done on horseback, with a nameless horse.
At least it keeps you out of the rain.