We used it, just got back last week.
We arrived on the Saturday before Christmas, I figured it was going to be crazy busy, there were 6-7 flights coming in every hour. They met us at the gate, checked our paperwork and escorted us to the front, out in under 15 minutes. I ran into people that told be they waited an hour or more.
On the way back, there was no line at all at immigration, but the lounge was worth it, free drinks (had to get one more rum punch in), patties and other snacks. A very civilized way to travel
I only tipped the bartender.
My advise would be to wait until a few days before your travel date, check flightstats.com to see how busy the airport is and act accordingly.