7.4/10. An enjoyable lark of an episode. The premise of Barney being hungover and everyone scrambling to try to revive him with a miracle hangover cure create some nice momentum for the episode, and the conceit of Barney only remembering a few things from that time, with a flash of images (like the roaring bear) and then the show slowly explaining how those images happened had some nice structural elements to it.
Ted eating bacon for the first time to get a container of grease and turning a little into Gollum was broad but cute. Lily and Robin's attempts to revive Barney were equally cartoonish, but generally fun. (Though the whole making out thing felt kind of gratuitous and too fanservice-y to me.) Plus all the call forwards to the various members of the gang breaking their vows to never get that drunk again were pure HIMYM-style fun. (I especially liked Lily's "you son-of-a-me" comment.)
There was even some nice writing at the ending, with the realization that the hangover fixer elixer was a placebo, but that it had been offered to each of them at their lowest points to help them feel better, and that lying to the passed out groom about "Weekend at Barney's" was the best way to return the favor. It's a canny little narrative move, and tying the whole thing into giving us more detail about and scenes of Ted and The Mother's future life as young parents was a nice touch too. Overall, some dumb-if-amusing humor but some nice writing to keep the episode going.