Honestly, I hated it.
Apparently, the ending had already been filmed back in like the second season, and my god, it shows. It seems to completely throw out all of the characters' development and growth over the course of all these years. It seems like a really soured attempt to push "realism" in a show that, while it has its share of touching moments, is meant to be a much more joyful affair.
Personally, I think they should have stopped at Ted meeting "the Mother", and just wrap it up from there, giving all of the couples their own "happy ending". More than anything, though, I thought their treatment of Barney was kind of shitty, completely reverting him to his more sleezeball incarnation, before forcing a child upon him, in a vain attempt to give him a "happy ending" that just didn't fit for the character. If anything, the scene where Robin and Barney wake up in their neighbor's room, tending to the child, should have spurred them into adopting, as I think that would have been a more satisfying conclusion.
As for the Mother, Traci; she was just poorly handled throughout the season. She didn't act anything even remotely close to "human". She was too perfect, solved everyone's problems, and was basically just "female Ted" in personality. Aside from looking incredibly young, I had no real qualms with the actress herself, though. They just needed to take the time to flesh her out as a person all her own, really incorporate her in the story, and maybe have her and Ted initially dislike each other, and spend the season showing how they came to love each other.
I also sort of feel like they could have done a bit more with Marshall and Lily. They went to pretty absurd lengths to let us know how shitty their lives have gotten; that's a fuckton of baggage to throw at people in the FINAL GODDAMN EPISODE, when really, we're just hoping to see how awesome everyone's lives wind up being. There is also a big deal made out of "everyone drifting apart", which again, is a depressing notion for a show about *friendship*, but moreover, it didn't even make sense for them to all be drifting apart.
This last season was a bit of a drag, but I still really like the show overall. The finale just kind of soured the overarcing story I think, though. Definitely one I'm going to mentally block-out, anyways :P