I'm not one for sit-coms, so I didn't watch "How I Met Your Mother" during its 9-year-run, but being the avid Entertainment Weekly reader that I am, I know something about the show. So I was mildly curious about how it would end, though not enough to tune in. And what's fascinating is how many fans (based on critics' reviews and comments added) hated it. When I saw "hated," I'm not exaggerated. I've seen a lot of comments to the effect that it ruined the entire series for them. TV critic Alan Sepinwall does a great job dissecting all the problems with it.
A common theme among the criticisms has been that the showrunners knew what ending they wanted, but it was no longer suitable given how characters had developed over the course of the show. However, they had already shot the footage of the kids' reaction to the story way back at the end of season 2 to lock in the age of the actors; there would be no way to film a scene now with 27-year-old Lyndsy Fonseca trying to look 16 (which she could pull off at age 20).
Of course, they could have just abandoned that footage and adapted the ending without the kids' reactions, so this seemed to be an instance where the producers wanted a particular ending.
Now, a number of long-running serialized shows that I've watched have, to varying degrees, fallen apart by the finale when it became clear that the producers/writers had been making everything up as they went along and thus couldn't possibly tie everything together without contradicting the past or retconning. So in that sense, one has to admire the HIMYM producers for knowing how they wanted to end their series and getting there. On the other hand, as the criticisms persuasively argue, that ending no longer made sense given what had transpired over the past few years.
Well, I guess that means we can cross HIMYM off the list of possible best series finales. I think I'm going with "The Shield" if I have to choose.