Many people will tell you to avoid it like a plague....saying full auto...not much more powerfull than on-board flash.... but why do I use it?
I trek often: jungles, mountains....etc. Without porters. I carry my own gear, camera and all. Many trips I find I can make do with a 220EX, granted it's a full-auto flash I find the E-TTL or even the plain old TTL flash output more than satisfy my needs.
Half the time I won't want to fumble with manual settings either....imagine crossing fast moving rivers, scaling cliffs.....I would more likely be using the other hand for support and such. It's the only bloody flash I can carry in my pocket while climbing.
Given a GN of 22....in practical use I find it about 3 times more powerful than the on-board flash, dont expect to perform miracles.... if I forsee I need more juice I will just lug the 550EX along.
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Do you need it then? If you are a streetshooter I think you can well afford the stength to carry heavier gear for a long time...a 550EX would double-up as a quick auto flash and manual settings comes along when you want it.
If you trek like me, you will want light and quick gear. I carry EOS300, 28-135IS, 220EX, Manfrotto SuperClamp (I seldom bring long lenses or tripod along).
Yes you see National Geographic photographers bring tons of L equipment everywhere....all kept in metal, shock-resistant cases. I can't afford that or the porters.
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market price? Last I heard is 200 over. Way way overpriced IMO...and that's why I love equipment collectors.... I bought a near-new condition for 100 only.