I assumed that the final season, even though it had been so bad, would at least provide a nice, sweet finale, but it managed to ruin that too.
I actually liked the short exchange between Ted and the mother the first time they met, but the rest was terrible. I didn't buy the Barney/Robin relationship to begin with, but it was worse to see the marriage fall apart in ten minutes, after having been subjected to a whole season dedicated to the wedding with so much filler, and then having the finale feeling very rushed. As said by others, there were sweet ideas in there, but the execution was terrible. If the wedding had been last season, or at least limited to the start of this one, more interesting ideas could have been explored, and the season could have been a lot better.
Finishing a show like that just ruins it and ensures that people won't go back to it. Many other long-running comedies where the quality really dropped off at the end have at least provided a fitting finale, and will be remembered fondly. Take Friends, Scrubs (season 9? I have no idea what you're talking about) and Futurama for example. Similarly to HIMYM, they should have been a couple of seasons shorter and got increasingly poor in later seasons, but the finales were really well done, so I'd have no problems returning to the show at all. This on the other hand, has killed any interest I had in returning to previous seasons or following the creators' work, which, when you're launching a spin-off of the show soon, is a terrible way to go.