As someone said on Twitter, the way it ended makes the entire series look like a huge con job, as a huge part of its appeal was the feeling that everything would be Okay despite all the hardships, as we knew that Ted would his true love and get his happy ending.... At least we thought we knew that.
Also, I read an interesting article about how this finale was another piece of evidence that TV writers have become addicted to making unnecessary twists in the final episodes. When you look at how Lost ended, how Sopranos ended, how House ended, etc, it all seems like the writers tried a bit too much to be smart (actually, I thought Sopranos ending was class, but no doubting it was a bit out there) and either failed to provide satisfactory explanations or delivered endings that seemed out of place with the rest of the show.
Anyways, it was definitely a big "feck you" to the fans, given that the vibe the show has emitted for nine years turned out to have been a scam.