Surely this depends on which concourse you arrive at and which one you depart from? e.g. would anyone land at a B gate, get the train to A just to use the lounge there if they weren't departing from A? (and vice-versa). It also depends on how long you have until your connecting flight - if you only have an hour or so you'd hardly spend time walking and on the train to go to another concourse.

I prefer the J lounges in A to those in B given a free choice but I only use B lounges when departing from B and A lounges when departing from A. I suppose there is technically just one A lounge which is spread out over the A gates but a few B lounges. I've only used the B lounge near B16 multiple times.