I work in the Barstow/Baker/Ludlow area. Although I've never ridden my bike on 66 I've driven it many times.
West from Needles to Barstow Rt 66 aka National Trails Highway parallels I-40 more or less. The road surface is ok in most places with long stretches where the top layer(s) of pavement have worn away leaving potholes of varying depths. From Newberry Springs to Barstow you may have to ride the shoulder of I-40 because I don't believe there are any surface streets available through Marine Corps Logistics Base Barstow. From Barstow through Victorville, Hesperia and Oak Hills (at the top of Cajon Pass) the route is sometimes off I-15 and sometimes you'll be on the shoulder of the freeway. Although this isn't fun the shoulder is wide and usually pretty clear of debris. BTW taking the shoulder of I-15 is FAR safer than traveling on US 395 which some people have considered. South from Oak Hills through Cajon Pass into San Bernardino you'll start on the freeway shoulder then move onto the old highway just south of SR 138. From there on it's a good ride. There's a short detour through Devore to avoid the freeway, then work your way back onto 66. From there to the ocean Rt 66 is almost entirely city streets...sometimes good route and sometimes a little hairy due to traffic, local environment etc.
There are very few services on 66 West of Needles; water is available at Goffs, Fenner (usually nothing at Essex), Ludlow, west end of Newberry Springs, Barstow/outskirts of Barstow, Victorville/Hesperia/Oak Hills, I-15 at SR138, Devore Food is available at Fenner, Ludlow, west end of Newberry Springs, Barstow, Victorville/Hesperia/Oak Hills, I-15 at SR138, Devore. In between those spots there is no reliable water/food at all. Unless you've been through that area you might be surprised how little 'civilization' is out there. Beware of "town" on a map that are actually just a collection of abandoned buildings.
Weather- May through Sep/Oct is brutal hot....days are usually +100F, nights cool off to the mid 80-90'sF. The wind is very predictable...usually calm in the morning then almost always from the West at 5-20 mph. I've thought about riding the Needles/San Bernardino section and am thinking it's better to ride the Needles-Newberry Springs section at night. There's hardly any traffic on 66 at night and as long as you have lights you'd be ok.