We've done it twice. We did not do the combo pass, but I kind of wish that we had. The ride behind Thomas is short, which was bad for two reasons. First, I'm just a big kid myself at Strasburg, and I wanted a longer ride. The Thomas trains do not do the switching operation at the end of the line that bresamil mentioned. Second, as a parent, the sooner you get off the train, the sooner your kids are immersed in the marketing blitz that surrounds the trains. They will want the toys & books, they will want to go on ALL the other rides and attractions and they will want it RIGHT NOW. A longer train ride, with maybe a picnic lunch in the middle of it, would help me avoid hearing the kids say "I want that!" more than a 100 times.
But aside from that aspect, it's a really fun trip. I loved Strasburg as a kid, and I'm impressed at how well they handle the huge crowd that comes for the Day out with Thomas events.