I've got two 580EX II's and a 430EX. If I'm using them on newer gear the 580EX II's are superb - however, if I have to use my backup 10D, ETTL doesn't work and it switches to TTL with shocking results. Either totally under or overexposed (mostly overexposed) - though shooting in manual on both camera and flash work OK. It isn't a huge issue for me as I'm about to do some camera reshuffling and retire the 10D, but if you have an older camera (1D, 10D or 300D) keep in in mind.
Build quality is so much nicer than the 580EX and 430EX - they feel like inferior products now - love the weather sealing, it makes shooting on location a little less stressful...