I have a compact double (50/34) paired with a 12/27 in the hilly SF Bay area, and while it gets me up most hills, I definitely see the benefits of a triple or a mountain cassette for sustained climbing. I'm sure I could eventually make it to the top of Mt. Diablo if I worked at it, but it'd sure be nice to have a triple so I didn't have to burn myself out trying to get there.

Still, it's a nice way to equip a bike for hills without the expense of converting from double to triple. That's ultimately the reason that I chose to go this route. And whatever doesn't kill you...