Free liquor was enough for me to pay for a NW lounge membership for years. I let it lapse once my flying patterns changed, but I miss it so much on my flights these days that I'm going to buy a new one as soon as I need it.
Chief benefits:
1) (Relative) peace and quiet
2) Free alcohol (not in UA lounges)
3) Free snacks (crappy, but still)
4) Knowledgeable/helpful agents who can help with misconnects, etc.
5) Access to international lounges which are really worth the price of admission