Yes, there are lots of people and the roads can get backed up. But there is traffic control so it moves along pretty well. Also, I've never stood in line behind more than a couple of people to get into one of the venues. Things are also so well spread out at the venues that it doesn't seem crowded at all, even though there are a lot of people there.
Marburger farms is the best for serious antiquing. They charge an entry fee, but it is worth it if you are looking for quality only. Warrenton is more fun, it's close by Marburger and it's more like a treasure hunt. (Free parking and entry). Be sure to eat at Royer's Round Top Cafe, they set up a location in the middle of the Warrenton venue. Every night, the exhibitors have a blast at Zapp Hall with live music and adult beverages. I've overhead some pretty great stories. If you are staying nearby, try to stick around for the party at Zapp Hall. One night is even "prom night," LOL. It's an easy day trip for me, so I've never stayed overnight.
I would also recommend checking out Cole's -- lots of great stuff, free parking and entry, also air conditioned if you have the need to cool off, and real restrooms (no portapoties). There is also Rifle Hall, which is another one-building venue, they charge a small entry fee.
Warrenton Antiques Show dealers
There are lots of other venues that are spread out and small, and some with more of a craft-fare type atmosphere, but IMO the ones above are the best for antiquing.