Only tend to use BHX in the UK, so I can only comment on that airport. I have had no problems, when they're actually in use that is! Recently, when arriving at a peak time, I found it quicker via the kiosk queue then waiting in the e-gate queue. The problem with the e-gates when busy is that you have to choose your lane and then hope that those in front of you in your chosen lane don't have any problems. My experience is that processing time through the e-gates generally takes a minimum of twice the time it takes to go through a manned kiosk.