They're looking for guns, drugs and weirdoes. Since you don't have guns or drugs and you're not a weirdo, you have nothing to worry about. By the way, just like at an airport, checkpoints are NOT the place to wisecrack about guns or drugs. Keep a smile on your face and don't be a smart ass. No turns, no surprises, no problemo.
Road is excellent by Mexican standards and good by American standards.
Keep your eyes open for a few potholes, but there are no major problems. From Ensenada, Baja, take Highway 3 over the mountains to Mexico Highway 5. Follow it right into San Felipe. Every once in awhile you see a weird scene in Baja. Hey, they just got electricity in 1964.