Interesting, Paul. I checked the Dax volume for IB against figures quoted here from a range of other suppliers recently and got an exact match (except with MyTrack, which is hopeless). I've never done the same for other instruments though, so it's possible they are poor elsewhere.
I use Sierra, which is what I did the check with, and also run my own software. I have checked a sample of 1 min bars from my app against Sierra and they match OK, so I'm happy they are both giving the same answer.