I did the T5->T3 transfer this week and it's horrible! When you arrive at T5 you have to use the horrid transfer bus to T3. It was packed with people standing trying to manage their bags. The route takes you the full length of T5 A gates, then back along the far side of T5 B gates before you enter the tunnel to Terminals 2/3. On arrival at T3 you are dumped a good 10 minute walk away from the Flight Connections Centre and mixing with other arriving passengers. (Interesting to note - no one has checked your boarding pass at this point so i could have continued to T3 immigration had I wanted to. Of course, any bags would have been delivered in T5) You then go down two floors to the new Flight Connections centre. There is a Fast Track queue and the normal line was very long. You then walk past the airline desks before entering terminal near the AA/VS lounges. All around 30-40 minutes and I am a fast walker. Would recommend avoiding at all costs.
How do you think it would compare to a land-side connection? Given that there are now more (new) immigration machines at T5, would exiting there and jumping on the HEx be any better?
I had to do that a few months ago when Heathrow staff wouldn't allow me to connect airside after I couldn't get an AA boarding pass. In retrospect it did seem easier. Exit Customs, on the Heathrow Express but then T3 security. Might even be faster!
I did the transfer again BA->AA at Easter. Horrible! Landed at the B gates and when the bus got there it was packed. Driver spent all his time slamming on his breaks and throwing people around. Got to T3 and as I was going AA had to join a queue to get an AA boarding pass. The line was not moving as AA had cancelled their flight to LA and everyone had to get home that day! After 20 minutes people not on the cancelled flight were sent to the lounge!
I've just done it in the other direction, and it wasn't too bad. The walk at T3 to flight connections isn't much longer than to the UK border, and is signposted. Lots of AA staff milling about trying to get you to collect your boarding passes, partly blocking the passage, but past them easy. I even ended up with a private bus to T5, after only a minute's wait, but that's just luck of the draw! I tried taking flight connections in T5, rather than going through the e-gates then customs then lift upstairs, partly as I needed a visa check and figured that made a slight difference on which is quicker, and partly as I wanted to try the new escalator up to south security (which I failed to...). That ended up taking a little while, as everyone in front of me at both the BA boarding pass check and at security seemed to have left their brains at home - best one was someone who put their coat in a tray to go through the xray, then tried to walk through the metal detector still pulling their rollaboard suitcase! Still, T3 gate 301 door open to T5 CCR in 35 minutes, not too bad, and certainly sounds a lot better than some of the experiences here going the other way
Security passes and regulatory checking are always a headache and after that I reached the departure lobby. Needed some peace at mind and rest for the body. Luckily London Airport transfer is in good posture and transport is quite smooth. Want to wave through that cab again as service was good.
I share your opinion, such situations are horrible. Queues are very annoying when it comes to such serious things as flights, as missing your plane is disappointing.