Bad or good ending?
THIS CONTAINS SPOILERS
For me it was an amazing ending. The fact that the mother died put a lot of emotion in it. You can see Ted's watery eyes (S9-E19 Vesuvius) when he's talking to Tracy in the Farhampton Hotel even before the "What mother is gonna miss her daughter's wedding?" part. Ted was almost saying goodbye to her knowing that she would die eventually. That for me was one of the most emotional moments of the show.
People forget something important, Ted is telling his kids the story about HOW HE MET THEIR MOTHER, not HOW HE CAME BACK TO ROBIN 6 YEARS AFTER THE MOTHER'S DEATH. That is why all season 9 is about just one weekend, the most important weekend in the story, because those were the days he finally met her.
And the fact that in the last 2 minutes of the last episode the kids convinced Ted to go after Robin, for me is a great ending because we all know that what Robin and Ted had was special. It's like, well, his wife died 6 years ago, he have to move on, and what better way than with the girl that was the love of his life so many years until he met his wife. Remember what Tracy told to Ted: "I don't want you to be the guy who lives in his stories. Life only moves forward".