Had the Sigma 500 before and was very happy with it but it was a bit big and also cheap feeling, the battery door "fell" off many times when changing batteries and I freaked each time but managed to wedge it back into place 
Cost only £150 and immediately was surprised by the build quality, the stand quality, the pouch..they all felt really good...
Seems to expose colours much more naturally than the sigma too, the sigma I had for both nikon and canon bodies (500 DG ST mkII) and on both systems exposed a more bluey toned image, the 430EX seems to give warmer tones when bounced off the white ceiling - both using ettl-ii and ittl for the nikon. I also prefer the vertical AF assist IR beam instead of the sigmas central beam which can miss subjects when you are really close.
Sample:
Room has a daylight energy bulb, this is where the sigma would give a much more blue tone and give more saturation to yelow areas, not the case on this! it's as natural as I have seen straight from the camera
Now for some lenses...

Cost only £150 and immediately was surprised by the build quality, the stand quality, the pouch..they all felt really good...
Seems to expose colours much more naturally than the sigma too, the sigma I had for both nikon and canon bodies (500 DG ST mkII) and on both systems exposed a more bluey toned image, the 430EX seems to give warmer tones when bounced off the white ceiling - both using ettl-ii and ittl for the nikon. I also prefer the vertical AF assist IR beam instead of the sigmas central beam which can miss subjects when you are really close.
Sample:

Room has a daylight energy bulb, this is where the sigma would give a much more blue tone and give more saturation to yelow areas, not the case on this! it's as natural as I have seen straight from the camera
