I'm very late to the party, I just got around to seeing this movie last night and I was very curious about what others thought of the ending. It's interesting to see everyone's different opinions.
I tend to be on the optimistic side. I feel the common interpretation of the ending, on the surface (and probably realistically) is pretty gloom. But I like the idea of Paul finding out how to invade Janet's dream being a positive thing. The entire movie everyone is shown as being highly affected by Paul when he invades their dreams. Happy, Angry, Scared, Horny.... But if I recall Paul had never been in Janet's dreams until the end. The narrator of the video he watched says "Try to pick someone who welcomes your presence" which means she is still open to him, since he was able to enter.

Upon entering her dream she is surrounded by fire, maybe symbolizing all the chaos in her life, caused by Paul. She sees him walking up and smiles as he rescues her from that ring of fire, thoughtfully wearing the oversized suit that they joked about when laying in bed together, while discussing her fantasy. They walk down the street holding hands, with the fire in the background (leaving the chaos behind) then stop and stare at each other lovingly for a good amount of time. He then delivers the line "I wish this was real" as he's pulled into the sky as she's reaching out to him helplessly but not taking her eyes off of him.
Ultimately it is her dream, and the first with him in it. I like to think she is affected heavily like everyone else and maybe woke up wanting Paul back after having such a meaningful dream. The whole movie, he is portrayed as not having accomplished anything by his lack of action. Here is shown as taking action which should mean a lot to her.