things change, new features make a product better, innovations provide new opportunities, and ultimately the latest and greatest provides the best online bragging rights- but with that comes a high price, so sometimes it's a better option to look backwards and see what was the top of the line a few years ago- sometimes that can give you better bang for your buck than buying the midrange current generation product.

The canon 550ex fits into that 'better bang for your buck' category. There are some improvements in the newer product line, but you really do pay for it, if you're just looking for light then the light produced by this flash gun is as good as anything produced today.

The name 550ex means it has a guide number of 55, the 430ex has a guide number of 43, and the newer 580ex's have a guide number of 58. So what does that mean? Well it means

"The guide number represents an exposure constant for a flash unit. For example, a guide number of 80 feet at ISO 100 means that a target 20 feet away will be correctly illuminated with an aperture of f/4 (80 = 20 × 4) using a sensitivity of ISO 100. For the same guide number and an aperture of f/8, the light source should be 10 feet from the subject (80 = 10 × 8)"
source: http://en.wikipedia.org/wiki/Guide_number

so basically the higher the number the more powerful the flash.

A more powerful flash will be quicker to fire, because the flash needs time to 'charge up' when it fires at full capacity, so with a weaker flash you will be firing at close to full power quite a lot, so you'll get slower flashing, and also more power is just better for when you need to bounce off something or throw a lot of light into a scene- something like a softbox can eat your light quite easily so the 430ex doesn't cut it sometimes. So if you're in the market for a new flash then this is well worth looking at. Read my 430ex and 580ex reviews for more differences between the 3 flash guns.