IB seems reliable, although I'm not on it all day.
Sometimes there are issues with particular securities, eg some hybrids were untradeable for a while (they would always appear as closed) and the customer service isn't the best (my referral to the 2nd level help desk seemed to disappear into oblivion).
But I don't think they get overwhelmed on a large volume day on the asx, probably because at that time the US market is closed and the system is built to handle US volume levels.
