EZ flash

Does anyone use an EZ series flash on their DSLR?

I'm getting the 20D soon and just want to know if I should use my 540EZ on it or sell it and get the 430EX or even a Sigma 500DG Super.

My understanding is that you need the EX, as the EZ is not ETTL compatible

Sorry, what I meant was does anyone still use their 540EZ in manual mode b/c they were so pissed that blew that much $$ on the flash, only to find out that in a few years it wouldn't be compatible with the E-TTL II metering system of the DSLRs. I don't understand why they couldn't have the DSLRs have two modes, A-TTL and E-TTL II, as a selectable option.

A-TTL still meters the flash light off the film IIRC, there is a preflash, but I don't know what that was used for. (A-TTL is not used on the Eos 1n / 540EZ when the flash was tilted). For some reason metering off the sensor doesn't seem to be possible with a sensor (although Fuji didn't have a problem with it in the S1).

I tried to use mine for a while. I became frustrated with it and gave it to my sister who still uses an older EOS SLR camera. I became tired of changing the settings as lighting conditions change. I guess it is all a matter of what you shoot and how often you would need to adjust your flash settings.

I bought a 550ex so I can not comment on the other flashes you mentioned.

I would sell it on ebay as fast as you can - it's just too frustrating trying to use the older flashes. It is a pity that Canon didn't make them more compatible. cynical marketting ploy? Not sure.
But I'd suggest sell it and get the 430 EX. The 580 EX is probably not needed by most people, and if need be if you upgrade later, the 430 will slave to the 580 just fine.
You might also want to look for one of the older EX flashes, I have the 380 EX which i got for my film eos, and it works fine on the 20D. Just stick it on the hotshoe, turn it on and go for it - comes out a treat.