8.5/10. I like both of the major storylines here. Marshall and Lily deciding whether to have a baby, and looking to signs from the universe, and realizing that whether or not they're actually seeing those signs is an indication of whether they're ready for it is a nice way to dramatize the internal concerns about bringing a new life into this world without just making it endless conversation. And the twist that Barney, who's mostly relegated to comic relief here, actually wants them to have a kid is one of those "heart of gold" moments for the character that help redeem him from the depths of his usual depravity.
Robin and Ted's storyline is very good too. In retrospect, the Don stuff is somewhat rushed (though perhaps in a fairly standard fashion for a sitcom), but the show does a good job at selling how Robin is really committing to this relationship in a way that she hadn't with Ted, Barney, or anyone else, and that makes it all the more meaningful and devastating when it falls apart. The idea that for once, Robin was putting love ahead of her career, and it came back to bite her in a bitterly ironic fashion is a nice bit of storytelling poetry, and while Ted's doppleganger speech is pretty facile and prior events for him and Barney don't necessarily fit the narrative, it is an interesting idea that despite the hurt she's feeling, Robin became more courageous over the years, and that's worth something.
Plus, it's also a solidly funny episode. Ted's blonde hair was fodder for lots of great comedy, from the always excellent psychic conversations among the gang, to Barney's list of one-liners, to the group actually liking it eventually, to Marshall calling him "Ellen Degenerous." In the same vein, the imagine spot of everyone being all up in Marshall and Lily's business about conceiving, only for it to come true later in the episode, was delightfully paced and acted by all parties, and even corny worldplay like "This just in...is what Marshall Erickson is about to say" had me chuckling.
Overall, it's up there in the pantheon of great finales from the show. It doesn't feel as epic as the dramatic stakes of the S1 or S2 finales, despite some major stuff happening, and yet that's a feature, since it feels more like just a particularly good, particularly eventful regular episode in its own way, which gives the episode its own charms.