Yeah, I'm scratching my head about the Tunnel Top. It's not a dive bar by any stretch - it's a bar designed to evoke a dive-bar charm for Marina dwellers that have ventured down to Union Square for a show, dinner, or a drink before heading home.
Then again, the bar offers a buck off their beers (Leffe Blonde, Beamish Stout, Sierra Nevada, Stella Artois were all on tap if I recall) during happy hour and $3.50 shots of Fernet Branca... I guess that's sort of dive. Oh, and they're cash only, that's dive as well. Actually, the whole bar is pretty well an authentic dive bar, except for the fact that the crowd was pure hipster who wouldn't venture into the Lower Haight even on a dare.
The bar gets a 3 star rating. Nice bartenders, well constructed cocktails and a very small selection of beer on tap ranging from Leffe and Beamish all the way down to the drek that is Stella Artois, the beer any self-respecting Belgian would hesitate to drink. Bring an ATM card because it's cash only (but they do have a bank machine). Make sure you have a fresh bottle of yuppie repellent if you want to have a real dive experience... but beware, spray it on too thick and you might be the only one left there.