I have used US Immigration at Shannon a year or so ago (very smooth) and I used the full pre-clearance facility in December 2009.
It is excellent. The Immigration (and now also combined as customs) officers are polite and very friendly, generally wishing you a loely holiday/trip, etc.
For Continental flights, you have to drop your baggage off at beween 6-54 and 7am, clear security and then clear immigration (which s open 7-15am until 7-45am). very fast and smooth.
Now that most trans-atlantic flights from Gatwick have moved to LHR, I find it easier and cheaper to fly to Shannon the night before and stay next to the airport, rather than paying double to get to and stay at Heathrow.
Do remember that Shannon is only so smooth because it has so many extremely low risk passengers and they have fully vetted all passengers long before they have arrived at the airport. It is amazing for connecting flights, as you arrive domestically.
Continental are starting afternoon flights as well for the summer - 2 hours 15 from check-in to take-off, and off the plane with bags standing outside the airport in Newark in 20 minutes, not bad!