Over the past nine seasons, the show has dealt with some heavy issues (Lily leaving Marshall, Marshall's dad's death, Robin not being able to have kids, Ted being left at the alter), so the fact that the show leaves on a "heavy note" (death, divorce, changing friendships) shouldn't be a surprise. It has never been a traditional sitcom, it has always been more part sitcom/part drama, which is what made it unique.
I agree with those who say the episode felt rushed, and some of it seemed forced (especially the stuff with Barney), but I honestly do not have an issue with the mother passing away or with Ted and Robin getting together.
People hate the fact they killed the mother, which is a testament to the quality of the show's writing and her acting (and she is brilliant...we saw her on Broadway in Once as she was awesome).
People all day have been saying that you do not introduce a new main character (the mother) into the story when you are 80-90% through it...well that is why I enjoy the show. It doesn't follow the "typical" rules...if I wanted a typical sitcom wouldn't be watching a show that has been told in flashback since it started.
People feel cheated, but in the end Ted is happy...isn't that what we wanted? Especially since he ends up with Robin because of a situation out of his control. Alright...that is all...now I need to start looking for a new show to watch.
On a side note...I would love to one day develop and write a stellar television show like How I Met Your Mother, but my gosh I am not sure I could handle the pressure of trying to write a series finale that people would enjoy. Looking back, there have been very few finales that everyone can agree on as being quality...most have their fans as well as their many, many haters. Kudos to Carter Bays and Craig Thomas, co-creators of the show for not only their what I believe to be excellent series finale, but for their nine-years worth of quality shows. Thank you gentlemen!