I actually quite like T5, going through the fast track to the lounge has generally never taken me more than 15 minutes. I've never had a problem getting through security, but then I normally use the one right at the south end.
I like the BA First Lounge, BA staff are very helpful and there are a lot more of them.
The T5B Lounge is like a ghost town, on the few flights I've flown from T5B its been lovely and relaxing.
But then I dont go to the airport to shop, typically I want to get in the lounge as fast as possible and chill out.
It does seem to take a while getting back in, imho Iris is a complete waste of time, it seems to regularly break down. Is it my imagination or has UK Border control slowed down so that we now think they are really going to clamp down on illegals? Good spin, but hardly worth while!!!