An external flash is one of those things many people think only a "pro" has a need for -- like a tripod. Once you get one, though, you realize how essential it is and couldn't imagine going into the field without one. I recently purchased this as part of my Digital Rebel package and am quite impressed with its features. The E-TTL -- the ability to adjust to the scene according to the camera's metering results -- is really amazing. On the back of the unit is an LED display showing your approximate focal length (24 - 105mm), which also determines how powerful the flash will be. Additionally, there is a pilot button/light (for a test flash), exposure confirmation light, high-speed sync switch, "slave" setting buttons and the power switch. After its flash flexibility, the 420EX's most impressive feature is its physical flexibility. You can move the flashhead within 90° vertically and horizontally, and it will lock at several angles. This enables the bounce-flash -- assuming you have a reflective surface nearby. Bounce-flash softens and distributes the light so you don't have overexposed, stiff looking subjects. Speaking of which, I'd like to say that I bought a Sto-Fen Omni-Bounce diffuser when I bought the 420EX because it seemed like a good accessory. I have found that I generally don't need it because the 420EX is able to fine-tune itself to each shot. I can't comment on its functionality as a slave because I have not used it in that capacity. Ultimately, the 420EX is a great tool, whether you're shooting your kid's birthday party or a macro of a flower. It will allow you to take much better photos than you can with the built-in flash. And at its current price it is absolutely unbeatable.