I hate going through preclearence. For a simple one-hour flight from Toronto to Dullles, I need to get to YYZ 2.5 hours before a flight - minimum - since you never know how long it will take to get through ICE. I've done it in less than 5 minutes once, but it's taken up to an hour.
I would much rather go through ICE at Dulles where I know that, regardless of how long ICE takes, I'll be home 15 minutes after I leave Customs.