Depends on your requirements. What exactly do you need to shoot with flash? Fill flash for outdoor photos? News photography? Multiple-flash lighting setups? Wildlife? Your cat? :-)
The Canon 430 EX flash is much less expensive, and works pretty well under most circumstances. The only serious downside, I think, is the lack of a port for an external high voltage battery pack. Yes, it's slightly less powerful than the 580, but I don't think that's something you'll notice except in extreme conditions.
I agree with Bud in that I've gone back to an all-Canon setup for small flashes. I use the wireless controller to fire the flashes (a 550, a 580, and 2 of the new 580 IIs), but I have all the flashes set to manual power output. I'm Old School enough that I expect the flash to put out the same amount of light every time I hit the shutter button, and I don't want no stinkin' camera thinking for me <big grin>.
You can probably pick up the Version I of the 430 or the 580 for less money at Adorama or KEH is good used condition. I actually prefer the original 580 over the new one -- it's smaller, lighter, and the Master-Slave switch is an actual physical switch, making it much easier and faster to use.
Hope this helps.
--Ken
