We have pass benefits on all three majors.
We take lower boarding priority than major airline employees and their respective wholly owned regionals. That being said, I've yet to have a travel plan disrupted by this. We just walk to the next carrier and get on.
If you've signed up for benefits with a major, you can check flight loads whenever you want.
You can put immediate family members on all of them. Delta is the best for immediate family because it only costs $50 per year for the whole family. Spouse/kids travel for free on domestic flights. Parents will pay a segment fee that's usually pretty cheap.
United allows you to also designate one person as an additional friend, who pays segment fees and for whom you are taxed imputed income on the Y fare when they fly. This is the only benefit on which you can put anyone you want.