Right, I've made a personal rule: I will try anything once, but if I'm going to drink any ale with "Butt" (or any synonym thereof) in the name it had better be phenomenally good beer.
Leinie's Big Butt is pretty good, unfortunately it is seasonal and only available January through March.
As an aside - Fat Tire and Shiner are overrated, crappy beers...
Blasphemy! Shiner Bock is one of my staples and my current fall back beer when selection is limited. Although I've been mostly unimpressed with their other offerings.