I would also like to know the answer to this - and how well the E-TTL
works with multiple Speedlights.

IR functions?
Do you mean the red focus assist?
If so...it works splendidly.
The wireless stuff?
If that's what you mean...it's wonderfully effective.
Certainly you don't mean infra red photography...
-Can't help you there.

I've used two 550s with great success...until one of them dropped 90 feet or
so to it's death with my D30.  I haven't replaced the second unit yet, but
still use the other and love it.

I used them both for macro on a wide bracket.  I'd wire one, and fire the
other remotely controlled by the first.  It worked like a charm.

NiMhs work great...especially the new 2100-2300mA ones.
They not only last longer than Alkalines, but they actually recharge FASTER
than new Duracell or Energizers, etc.

This flash is way overpriced for what it does. You can have a better
dedicated flash system for your Canon if you buy multiple Sigma EF-500
Super TTL Flashes. Canon EOS lenses and flashes have been known to
have numerous compatibility problems with Canon DSLR bodies because
they were designed to work with film bodies. Sigma dedicated lenses
and flashes work amazing with camera digital bodies because they were
reverse engineered just for that purpose.

If you really want the best images possible, lose the sh.t pail Canon
system entirely and go all Sigma for the ultimate image quality,
optics, lens/accessory selection and compatibility, wireless flash
system, build quality, and pro features. Get the Sigma SD10.

Yes.

Works as advertized.

NiMH's are the best, recycles within 6 seconds after full power
discharge, and they don't suffer from memory effects, so you can
recharge at any time/charge level before you start a session.
Total number of flashes depends on capacity and exposure conditions.

Bart