For ordinary use, the 430 EX II is fully adequate.
It's for more demanding applications you may want a 580 EX II. When shooting sports at night, for example, I appreciate the 580 EX II flashes.
This is with 7D + BG-E7 + EF-S 17-55 mm f/2.8 IS USM + 580 EX II on camera (as a master and for some fill), and then another 580 EX II + CP-E4 as a slave on the other side, as a main light.
Note that the built-in flash controller in the 7D isn't capable of more than a few shots in succession, then it needs rest to cool down. So it's not able to control slaves in action situations, where you don't know in advance exactly when you need to shoot. Nor is it useful for portraiture, where at least I, who aren't very good at it, needs to take a number of images in quck succession, to get something good out of it.
For night sports like this, the CP-E4 battery pack makes quite a difference in performance. The 430 EX II isn't compatible with external battery packs. But again, this isn't ordinary use.