I've only used the new system a couple times, both late in the evening. Both times there was no line, and I went from plane to getting off the bus at the domestic gates in under half an hour. It was significantly better than the old system. I've never had to wait for the bus to fill up, and it seems they just go after a few minutes regardless of how full they are.

That said, I do wish they had NEXUS kiosks. I imagine that during peak times, it can get quite busy.