here's a little example for shits and giggles. difference in guide number. bear in mind this little formula:
guide number / distance to subject = suggested fstop. (we'll use 10ft as an example because it's the easiest to calculate).
with that in mind. the built in xti flash has a guide number of 43 (max), which means that 43/10= f4.3 ... not bad... shallow
the 580ex has a guide number of 191 (max), which means that 191/10=f19.1 ... now, i don't know if you, personally, know the difference, but it's almost 5 full stops.
get it?
not to mention the ability to zoom my flash to either diffuse it or concentrate it.
yeah, well worth the money.