The 580EX-II was the top of the line Canon Speedlite until the release of the Canon 600EX-RT. For many shooters the release of new equipment opens opportunities for them that had not been practical before as older equipment re-enters the market at 1/3 or even 1/4 of original cost. For many shooters this will allow them the option of adding a powerful E-TTL II flash to their kit that can also function as a master controller for their existing 430EX-II or other Canon Speedlites.
Many other shooters will be wondering if the 600EX-RT is a better option. Lets take a brief look at that first. Both the 580 and the 600 are capable as functioning as an optical master for the existing Speedlite lines, while the 600 is capable of controlling 5 flash groups and the 580 only controls 3 in optical mode the 600 is also limited to 3 flash groups. The rational behind this limit was probably limit of the older flashes to groups A thru C, however it would have been nice to have the option to use older units in those groups and use 600s in the D and E group in an optical controlled system. The short of it is to gain the added advantage of 600 series flashes at this point in time all flashes have to be 600 series making that set-up much more expensive then using a 580 master with other 580 and 430 units as slaves. Used 580s are now comparable in price to new 430s which makes adding a used 580 to the mix an attractive option.
For shooters that are already using cameras like the 60D and 7D which offer master controller function through the pop-up flash paying an additional $550 for a new 580EX-II was probably not an attractive option. Now you really should consider picking one up used in the $200 dollar range, this is why. The 60D and similar model cameras while capable as acting as a master controller were limited to flash groups A and B by using a 580 as your master controller you now have access to group C as well creating much greater freedom when using speedlites for 3-Point Lighting. For the higher end cameras that already supported all three flash groups the big advantage to having an on camera 580 is when you are controlling flash groups with the camera oriented to portrait format. The common problem the pop-up unit encounters in shooting in this fashion is putting some of the off camera flashes into the shadow of the lens which prevents them from firing. The larger surface of the 580 unit along with the ability to orient it in different directions can allow all of the flash units to easily see the master unit.
If you are using a higher end camera like the 6D a pop-up option is not available and a 580 is a very affordable option. While it is true that ST-E2 units can be purchased new in the same price point as used 580s, the big advantage of a used 580 is that it gives you one more flash unit. Even with this unit mounted on camera it can provide fill light or be used as a Bounce Flash.
The 580EX-II is still a very relevant option for most shooters. It is a powerful reliable choice and when it comes to the use of other optical units it’s price point makes it a superior choice to the 600 Series.