I've been through T3 a few times, never any other terminal though.
Been to and from LHR using both Underground and coaches. Travelling both with check-in luggage and without.
Yes it's bloody big, it can be crowded. But I don't think it's that horrible really.
Immigration lines haven't been too bad, around 10 minutes average for me I'd guess, which is not to bad.
Security has always been fine.
The only downside is the lack of seating out of the main area (I prefer to sit by the window and look at planes to shopping), but sometimes you can find an empty gateroom and stay there for a while and that usually works fine.
There are plenty of signs and info screens and as long as you can read and follow arrows finding your way is quite easy (according to me anyway).
Sure it's big, and sure it's crowded, but that bad, no I don't think so. To be honest I think it works quite fine.