Like anything in Norway, the trainride is expensive. A roundtrip will set you back about 50 euros.
I was staying in Flåm with a group of friends and we were recommended the train as THE thing to do whilst there. But all it really is a trainride up the mountains and back. It takes you two hours and the photo-ops are limited unless you score a seat by a window that opens.
If you're travelling in Norway by car, don't bother. There are a lot more scenic views on the road than on this trip.
SPOILER ALERT - don't read if you want to be surprised
There are these ridiculous mountain spirit dancers waiving around about halfway through when the train stops by a waterfall. It's absolutely pointless and I don't see how anyone would find it adding to their experience.
When in Flåm, I suggest you tour around the village, go kayaking or walking across the valley. Feel free to skip the train, you won't be missing out.More