Join Date: Mar 2015
Location: NYC (Primarily EWR)
Programs: UA 1K / *G, ALL Accor Diamond, IHG Platinum Elite, Marriott Bonvoy Gold, Avis PC, Hertz PC
Posts: 10,663
If you're based out of SFO, I feel like there is plenty of nice space in the terminal to sit down and relax at. That said, the lounge at SFO (the older one, not the new one) is my favorite domestic UC, and I have heard good things about the Boston one as well.
YMMV, but if you want club access, why not just sign up for the MileagePlus Club card? Might as well get 1.5 miles per $1 spend on everything while you are at it...