Currently, my high is a 44x12. That works out to be over 30 mph so I'd need a pretty good tailwind to need more than that (or a downhill). My low is a 22x27 and I used it last week.

A mtb triple makes a great compact crank for a road bike. About all I need is the middle ring most all of the time as it works on every cog so that alone provides 10 evenly spaced speeds. However, it is nice to have additional resources.

I have a friend that has compact cranks and a big hill on a great local ride that would be very convenient for him and his Madone; however, it's too steep. He takes a longer and more boring hill around the big one but even the lesser hill is still pretty steep for his gearing. He could easily spin up the steeper, shorter, more scenic, less traveled hill with a lower gear.