I had the Quick Adjust Messenger strap on my Ego and it was comfortable but I switched it out for the Absolute Strap and the Absolute has been on the Ego ever since. The QAM was comfortable but the Absolute was even more comfortable. And I didn't adjust the QAM length/position enough to warrant keeping it on when the Absolute was just that much more comfy. I still have the QAM in case I change my mind but I should probably just put it up on eBay.
I use the Absolute strap on my Western Flyer. I also usually have problems with straps slipping off my shoulders, but the Absolute strap is very grippy. It is wide, but given the weight of my packed WF, I think it helps distribute the weight and makes carrying the bag easier. BTW, I have the WF with the roller-board pocket because I wanted the extra external and internal pockets that you lose with the backpack straps.
On the other hand, I use the regular strap on my Imago and Ristretto. I've tried the Absolute strap on these bags and do find it too big and bouncy. Amazingly the regular straps don't slip; the grippy shoulder pads on them really work well.
I prefer the backpack straps on my Aeronaut, unless it's a very short distance, in which case I just use the handle. Probably the only advantage (for me) of the shoulder strap is that it makes me look 7% less dorky.