I never have - terminals and their amenities these days are so nice, especially when my home was SFO. That and I can usually fly nonstop, so I'm rarely waiting for more than 10 or 15 minutes. Perhaps if I had to regularly connect.

Now that I'm with Delta I have SkyClub access, but only because it's a tag-along freebie on a number of Amex cards, whereas UC membership isn't available through any method that doesn't involve substantial direct cost.