Why is anybody confused by the ending? It's very straight forward.
I'm reading these threads on imdb, and it seems that a large portion of the people who've watched this movie are posting here because they seem to think that the ending necessitates interpretation. There is no need for any of this. If you're confused, watch it again. I find it to be very straight forward, without much room for any attempt at interpretation or interpolation.
The Ending:
1) Rhoda from Earth 1 gave her ticket to John after telling him about the theory that the parallel Earths diverged timelines 4 years prior, upon first discovery, before the car accident.
2) John accepts her ticket to travel to Earth 2, hoping to find the doppelgangers of his family alive on Earth 2.
3) Rhoda from Earth 2 keeps her ticket and travels to Earth 1 to meet her double, because she had nobody that she wanted to give it to, unlike Rhoda from Earth 1. This tells you that either the accident never happened on Earth 2, or if it did, perhaps John from Earth 2 turned down the ticket when offered.
4) In seeing that Rhoda from Earth 2 has traveled to Earth 1, and is in considerably better shape (happier looking) than Rhoda from Earth 1, it is implied that the accident never happened on Earth 2, and John has found his doppelganger family on Earth 2, where he will stay and live out his happy life.
My Commentary:
The two things that I find funny about the end:
1) If John from Earth 1 did find his doppelganger family alive on Earth 2, John from Earth 2 is likely still in the picture with them, therefore leaving Earth 1 John alone again.
2) Also if John of Earth 1 finds his doppelganger Earth 2 family alive and well, they still aren't his actual family. They are merely twins from a duplicate planet. They're not the people that he loves.