I have been using IRIS since 2007. I enrolled at Gatwick when American flew to DFW from there.
There was a bad spell in 2009 when T3 always seemed to be playing up but the last couple of years T3 has worked a charm for me and, as it is T3, the bags are always ready waiting.
Nice quick exit which is perfect after long flight and usually 30 minutes going round in circles before landing and 20 minute wait for a gate as someone else blocks the alley at T3 or waiting for a bus at T5.
Not linked to Global Entry like Dutch Privium is but give UK Govt another 10 years and they will probably get round to it.
Very upset to read above they are pulling them for ePassport readers.