Well... let's start with the obvious. I think this episode is going to generate a lot of mixed feelings, at least based on the most "essential" plot point (if you want to see it that way). The scene at the end of episode 12 meant what it looked like it might have meant: Yasuna broke up with Hazumu, and they're back to being close friends. The second scene you saw at the end of episode 12
also meant what it looked like it might have meant: Hazumu finally crossed the river completely and confessed to Tomari. You'll probably remember what exactly that means/implies. The storytelling purpose in all this is to, of course, bring balance back to the equation (sort of).
As an episode, it's probably most helpful to think of this as the OVA/bonus episode that it is. While it is the end, it's hard to say that it's the "be all and end all"; thematically, episode 12 was the "ending", and this is sort of like the epilogue. The episode itself is something of a wrap-up for all the characters, with all the major points from the series being brought back in some form. Even the famous perverted dad and single teacher skits make their reprise - but, perhaps because it's been so long, they didn't seem as annoying (and they were over quickly - it was more of a nod than anything else).
Plot points... the basic story is that Tomari is keeping her distance from Hazumu after his sudden confession. Obviously she's a bit confused - just as we saw in the show itself, he just finished the big dramatic Yasuna ending... how could Tomari not feel like "second best"? Hazumu tries to invite her to Christmas events (including Yasuna's Christmas concert, and an after-party back at his place), but Tomari keeps avoiding him (and skips out on the concert and party due to "work" - read: excuses). So, Yasuna and the rest of the gang (with help from Jan-puu and "Sora-sensei"), knowing this would probably happen, already came up with a plan, and "kidnapped" Tomari and reunited her with Hazumu. The result was a bit of a Christmas confession spectacle befitting the show's grand finale and also wrapped up Tomari's plot in the show.
So, back to impressions... as much as I might otherwise be upset that they screwed over "Yasuna's ending"... it's actually somewhat befitting the show and the continual back-and-forth that we saw. It cleans up all of Tomari's loose ends, just as episode 12 cleaned up Yasuna's. I don't take the "new Tomari ending" as being the real end for these characters either (despite the "final-seeming" look to it); I think this was more like bringing the balance back. The episode was very much a "reprise" of the show (as opposed to a straight continuation), and was careful to reflect back on all that had happened, so it felt nice to come back to it after so long. As a bonus episode, reprise, and grand finale, I thought the episode worked pretty well - but it also makes perfect sense why they never aired it as "episode 13", because 12 really was the end from a over-arching theme/storytelling perspective. In that sense, it's exactly like almost all the other Bandai Visual "episode 13" OVA episodes, so it makes sense.
