We did it last month. We arrived around 1 on the Saturday before Christmas. They met us at the gate, checked our paperwork and then escorted us around the very long line right to the front of the VIP line. Same in customs. They helped us find our bags and escorted us to the front. Out in under 15 minutes. I heard from others that the wait was well over an hour that day.
From other's reports, most have said that the line is usually very quick, especially mid-week.
Flightstats.com only gives you 2-3 days worth of info, but you can purchase the arrival service on fairly short notice.
Surprisingly, on the departure day (1/2), there was no line at immigration. The lounge is really nice though, we had several drinks and filled up on patties and other snacks. A very civilized way of spending a few hours.