I registered for IRIS on Monday afternoon at LHR T1.
When I turned up at the registration room at 14:50, there was a sign on the door 'Opening hours today: 15:00 - 18:30'.
As I wasn't in a hurry, waiting 10 minutes wasn't a problem, however, for anybody turning up earlier in the day, they would not have been able to register.
When they opened, I was the only customer there, so no waiting time and I must say the staff where friendly and pleasant, in all it took about 5 minutes to register.
Arriving back at T1 on Saturday afternoon, there was no que to EU/EEA passport control, but I thought I'd give IRIS a try.
When I got to the machine, there was one person in it and one other person waiting to use it.
The person inside (a late middle age american gentleman, of a confused disposition) spent at least 10 minutes in the machine trying to get through and then back out again.
It turned out that he hadn't registered for IRIS, but thought that in todays 'Big Brother' world, the authorities already had all information about him on file.
For the person in front of me and for me the process worked and took less than 30 seconds each.
What I would say is, that one machine can never be enough, as with only one machine, one person or one piece of faulty equipment can hold up the entire queue for quite a while.
I think that in future, I'll only use IRIS if the queues to EU/EEA passport control are long.
bjorns