With daylight fill/bounce flash I'm mostly shooting Av mode with fixed iso 800-1250, min. shutter speed set (1/60s - 1/250s on my 60d) and a little negative flash ec to prevent overflashing the subject or positive flash ec because the diffuser eats too much light. I'm also often using camera ec to control the subject-background relation, so most of the time it's set between +1/3 and +1 to allow for a natural background.
The weird behavior I am experiencing: When changing *camera* ec a little from 0 to +-1/3 the background exposure changes a *lot*, while the following 1/3 steps are relatively minor - so on my 60d+600rt camera ec is not a constant, gradual scale. Is this just me, my camera or my flash? Is the flash exposure depending on the camera body used, or do all the ettl2 Canon cameras meter the same?
There are a few things that can make ETTL act a little weird or even inconsistent. Not sure about that specific camera-flash combination but I've done a bunch of testing a while ago together with some folks over at dpreview. I'd have to go back and look at the details but two things stood out when using a 5DII with a 580EXII: one was that ETTL behavior when the flash head is straight or angled came in different (less so with my 430EXII). The other one was that it can vary with which lens you use. Some lenses transmit their distance settings, others do not. The ones that do become part of the "TTL" calculation - and sometimes that's weird. In a way you could even say that these lenses (my 24-105 for instance) under some circumstances actually leads to a calculation in camera that is not really "TTL" but rather the good old GN/distance thing. Lenses without the transmission of this information came in a lot reliable.
So when you're compensating these factors may throw things off a bit. Ultimately, that is one reason why I've started to use manual settings a lot more instead of relying on ETTL all the time; at least in environments where light doesn't change that drastically.
When using ETTL I also find that Av is not the best mode but that manual camera setting are really the best for that. Syl Arena mentions that in his book also - something to the extent that M is really the default "automatic" ETTL mode. I know that Av is always said to be for daylight/fill flash. May be worth testing if other settings work better for you.