Syl Arena, whose excellent Speedliting book is available through Amazon, recommends starting with a 430EX II if one's budget is limited. I have no personal experience with the 430EX, original or II. I do highly recommend the Speedliting book, and keep in mind that clicking through to Amazon via the link here on the Cameralabls site results in Cameralabs getting a small fee, without costing the buyer extra.
I was lucky to start DSLR shooting, using a 580EX, along with an XTi/400D, that my wife acquired pre-owned from a colleague. I subsequently acquired a Nissin Di866 II Professional, and a second pre-owned 580EX. These provided good service, but when I later acquired a new 580EX II, I did notice, over time, consistently better exposure performance, using E-TTL mode, with my 7D cameras in Manual mode, compared to the original 580EX flashguns. It is more likely I will feel a need to re-shoot when using an original 580EX. A pre-owned 580EX is still going to be a quite good flashgun, for one who has a need for a powerful flash, and needs to save money.
I should note that a buyer should, if possible, thoroughly inspect a pre-owned flashgun before purchase, as some will have corroded battery compartments, due to neglectful owners.
The only drawback I noticed with the Nissin was some white balance problems when shooting in mixed artificial ambient lighting, outdoors, using AWB. (The images were too warm, with an orange tint.) When I switched to a Canon flash, which was either a 580EX or 580EX II, the colors in the image were
true. These were evidentiary images, which I had to submit as straight-out-of-camera JPEGs, so getting white balance as correct as possible is important. I still find the Nissin useful, but consider my 580EX II worth its price, even though it strained my budget when I bought it. Those who shoot RAW can correct
white balance, to an extent, when processing the images in one's computer.
My wife has had good results with a Nikon-fit Metz flashgun I bought for her to use on her D7000. Metz makes parqllel flashguns to fit Canons.
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