Im a Registered Traveller and it is so hit and miss, such a waste of money. My one attempt at the E-gate failed miserably and I seem to get more questions from immigration than before.
I stopping myself from naturalising to British citizenship to keep my access to Fast Track immigration which seems to be the fastest queue through immigration most of the time.
Although, at Gatwick I got screamed and abuse by other passengers for skipping the fast track queue at gatwick which was about 20 deep because they have a separate registered traveller queue and they all thought I was just trying to skip. I showed them all what was what though :P and got a very sincere apology from the Immigration officer who said Registered traveller wasn't being communicated well enough.