I run a 50/34 with a 12-27 cassette which replaced a 52/42/30 triple with a 12-25 cassette. When you compare the 34-27 combo with the 30-25 combo you can see that it is pretty close but not quite as low. If you look at Sheldon Brown's gear calculator and compare the two set ups it looks like this:

30-21 = 37.5 gear inches 34-24 = 37.2 gear inches
30-23 = 34.3 gear inches 34-27 = 33.1 gear inches
30-25 = 31.5 gear inches

These numbers are generated with a 700x23 wheel/tire and a 172.5 crank for both combos. As you can see the 30-21 is nearly identical to 34-24 and the 34-27 is in between the 30-23 and 30-25. I have about 400 miles on my compact setup now and have found it to be very nice and the low gears are plenty low even to get over a big steep climb at the end of a long ride. Persoanlly I like the compact double better than the triple.