Summary: This flash is much better than the built in one from my DSLR. My camera automatically detected this flash and adjusted to the flash without any problems. You might want to get a diffuser as well. It uses 4 AA batteries and comes with a paded case. This flash upgrade is definitely worth it. Highly recommended.
Summary: I would say that this flash is worth every penny, and well worth the price premium compared to the 420EX. I was struggling to choose 430ex & 580ex. Their price are about $100 different. There are many functions on this flash that I don't understand and the manual didn't tell you much at all. I found that there is a Dvd showing you how to use this flash and I am going to get it. I am sure I can do many special tricks with this flash.
Summary: I am not a pro, but I am an enthusiast and have been using a film SLR since the mid 90's. This is my first external flash unit and I can say that if you have the budget for it - it is worth the purchase. This can make a huge difference in not only indoor photography, but outdoor as well (used as a fill flash). I have pretty "fast" lenses which are capable of doing indoor shots (canon ef-s 17-55 f/2.8 and sigma 30mm f/1.4) and I still decided to buy the 580EX II.
Pros: Quick recycling time, flash power, weather sealed...
Cons: Big/heavy, expensive, there is a learning curve to take nice flash pictures
Summary: This is a fantastic flash. It is pricey but I was able to get mine on sale for $100 less than regular price so it was a no brainer for me. This flash delivers tremendous lighting for my Canon EOS 40D. I love the fact that I can bounce the flash off the ceilings from every angle. My pictures have come out great and even the people at the camera store have commented favorably on my pictures as they are processed.
Summary: I have both canon 580ex and 580ex II. This latter version has faster recycling time, about 25% faster. one thing I noticed is the size of it is a little bit larger, my omi bounce hood can fit in 580ex properly but can hardly fit in ex II. the nice thing is it has PC terminal so that you can connect it to pocket wizard easily.