I think this is the worst ending I've seen. Dexter had already slid into shittiness so we weren't really invested in the characters anymore and while the ending was dumb and completely against everything they had built up to, no one gave a shit. Lost had declined as well, and the ending was disappointing, but at least it was one that you had to think about for a little bit to decide if you thought it was terrible.
HIMYM, however, was an instantaneous, "Are you fucking kidding me?!" At least for me, but I think plenty of others shared that opinion. The posts here and elsewhere on the internet immediately after the show were almost universally negative. While the show had been declining, at least you could point to stuff like Barney and Robin and say they had experienced growth as characters that paid off...until they just shit all over it with the divorce and then Ted going back to her.
I can find at least halfway legitimate reasons to not *completely* hate the Dexter and Lost endings. Dexter's ending is definitely very close, because of the same buildup of character growth that got thrown away, but I can't find a single reason not to hate HIMYM.
Both shows would have been 100% better if they stopped the ending just a little bit earlier. Dexter should have just died in the storm, and Ted should have been standing under the yellow umbrella with the mother. End them both there, and we aren't having this conversation.