I'm sitting in the LHR MLL now, finishing up my trip to London.

Overall T2 has been a pretty good experience, although it is still pretty empty since not everyone has moved in.

As others have mentioned AC is in T2B, so you have to take an escalator down under the runway and back up again to/from the main T2A (where most of the shops are, security & customs are, etc.). It is a bit of a walk but there are escalators/lifts and moving walkways so it didn't seem that bad. The walk in the old T3 wasn't exactly short either; there the lounge was closer but the hike out to the gate was longer. Departing priority security was empty (it is over on the right hand side) and I breezed right through everything so I get to sit in the new MLL for a bit.

Speaking of the MLL it is lounge B3 - there is no AC MLL sign until you are right on top of it when you arrive in T2B. There is a full breakfast spread out for the morning flight to YYZ and it is pretty good - better than FRA in my opinion. The lounge is not huge (smaller than FRA for sure) and I could see how it could get busy but with just the one AC flight leaving shortly it is pretty quiet right now. You can tell they used the same architects that designed the FRA MLL though, and it is much nicer than the old Ikea store in T3 that they shared with SAS.

My $0.02: T2 is much nicer than T3, overall a better experience in pretty much all areas, although we'll see how they do with queues and such when the terminal is running at full capacity.