I've been shooting with ambient for years but I've recently decided to get a decent external flash. I'm mostly thinking about getting a shot where you'd otherwise have to crank it to ISO-stupid, or for fill flash for portraits in the sun/other harsh lighting - I'm not really concerned about studio work at this point.
As a Nikon user it would seem that my two choice are:
SB-700 (~250 quid)
SB-400 (~ 90 quid)
Both have rock solid reviews, but obviously the 700 has a ton more functionality and specific controls, while the 400 is much closer to the sort of price-range I was hoping for. I don't know what the Canon equivalents are, but if you have an opinion on the topic I'm sure you can tell me.
Under what circumstances do ye find yourselves using your flash most, and what functions do you rely on? Or I suppose the most important question for me is: Under what circumstances am I likely to be kicking myself saying "Why oh why didn't I get the more expensive flash"? Or, the other side of the coin, is an expensive flash like that wasted on a non-professional?