Had 4 BA group boarding experiences recently.
MAN - 1-3 are a single line here (basically the old "Priority" method), because that's all there's space to do in the corridor. The Group 1-3 line had already cleared when I got to the gate. They hadn't started letting the Group 4+ line through yet though, as the jetway wasn't clear, so I just joined the short queue on the jetway.
LHR T5B: Groups 1-3 have seperate lines, and then everyone else. Boarding already in progress. Straight into the Group 2 line for me, a short wait, two agents were serving the 1-3 lines. If Group 1 was empty, that agent would service the Group 2 line until a Group 1 passenger arrived, then served them next. The other agent would alternate between the Group 2 and 3 lines. Moved quickly. No-one seemed to be trying their luck in the empty Group 1 line.
DFW: Not really enough space, plus the gate arrangement, and a flight that was late boarding due to a late inbound meant a bit of a scrum with people getting to the gate before boarding had got going. The separate Group 1 and 2 lanes were arranged facing the gate, while the group 3+ lanes (there were another 3) were perpendicular to the gate. The Group 1 and Group 2 lines were blocking the way through to the higher group lines. This meant people kept joining the Group 1 & 2 lanes and had to keep being re-directed to the right spot by staff. A massive amount of folk in Group 2, but this was an 86J 744.
LHR T5A Domestic (Auto-Gates): Clear signage, no-one seemed to be queueing in the wrong lanes. Boarding already in progress, walked straight up through the Group 2 line to an e-gate.
My opinion is that it seems to work better. Less scrummage, and clarity on who boards when because it's clearly marked on the Boarding Card.