This is an absolute do-everything flash, with master/slave operation, wireless control of multiple channels for slave flashes, A:B:C lighting ratio control (better than the ST-E2's mere A:B ratio), full manual or automatic operation, strobe effects at selectable Herz rates and power, and even a small panel that serves either to widen its flash coverage to 17mm or simply provide a catchlight in a model's eyes when using bounce flash.  Range is excellent, battery life is very good and can be compounded with one of the battery packs.  The light itself provides excellent color, and a red lamp near the base adds a much more accurate and less harsh focus assist lamp than most of Canon's cameras provide.

If you have an SLR, you'll almost certainly want an external flash.  You can provide fill flash to reduce shadows in subjects eyes etc. during harsh daylight shots, take the flash off the camera to eliminate nasty shadows, and avoid the shadows caused by larger lenses and onboard flashes.

Minor quibbles: most DSLRs have crop ratios of 1.3x or 1.6x, but don't pass this information on to the 550EX, which wouldn't care anyway.  This means the flash thinks you want greater coverage than you really need (i.e. it will give you a 50mm's angle of flash coverage, when you only need an 80mm's), so you waste a lot of flash power and don't get optimum recycling times.  Also, the interface is a little non-intuitive.

If you want a slave- or onboard-only flash and don't need all the extra features, the considerably cheaper 420EX is for you.  That said, if you do want all the extra features, the 550EX is still not for you.  You want the 580EX, slated for October 2004 release, which fixes the minor issues with the 550.
This is an absolute do-everything flash, with master/slave operation, wireless control of multiple channels for slave flashes, A:B:C lighting ratio control (better than the ST-E2's mere A:B ratio), full manual or automatic operation, strobe effects at selectable Herz rates and power, and even a small panel that serves either to widen its flash coverage to 17mm or simply provide a catchlight in a model's eyes when using bounce flash. Range is excellent, battery life is very good and can be compounded with one of the battery packs. The light itself provides excellent color, and a red lamp near the base adds a much more accurate and less harsh focus assist lamp than most of Canon's cameras provide. If you have an SLR, you'll almost certainly want an external flash. You can provide fill flash to reduce shadows in subjects eyes etc. during harsh daylight shots, take the flash off the camera to eliminate nasty shadows, and avoid the shadows caused by larger lenses and onboard flashes. Minor quibbles: most DSLRs have crop ratios of 1. 3x or 1. 6x, but don't pass this information on to the 550EX, which wouldn't care anyway. This means the flash thinks you want greater coverage than you really need (i. e. it will give you a 50mm's angle of flash coverage, when you only need an 80mm's), so you waste a lot of flash power and don't get optimum recycling times. Also, the interface is a little non-intuitive. If you want a slave- or onboard-only flash and don't need all the extra features, the considerably cheaper 420EX is for you. That said, if you do want all the extra features, the 550EX is still not for you. You want the 580EX, slated for October 2004 release, which fixes the minor issues with the 550.