when I first started out I had the 430 EX II, it worked and did it's job.
the bad thing about this hobby is that you'll only want better, and it won't stop until you buy the best version.
lol, it's a little cliche but yeah i ended up selling the 430 ex ii, and paid the difference to upgrade.
the little features like the spin wheel & how the 580 ex II lets you twist it's head all around has made a huge difference for me. It all depends on how you shoot though. better equipment doesn't always mean better photos, the features are there to help you save time
also the fact that you can always sell the stuff back on FM at little to no loss