4.5/10. One of the low points of the series. The entire plot is a feint to try to think that past history between Ted and Robin is going to be what scuttles the (rushed) wedding, while in fact it's Stella going back to Tony. The problem is that nothing that motivated it makes sense. Stella has hung out with Robin with the gang multiple times and never expressed any desire that she not be there. And we've never met Tony before, so he's just some random, goofy guy whom we know nothing about. The fact that Robin validates Stella's sentiments, when she and Ted have hung out together for a long time and it'd be, frankly a dick move not to come to the wedding (assuming she had more notice and didn't have to hop a flight to Japan) just because they have a history. It's all a huge development in terms of the characters and their relationship, and none of it feels organic or natural in terms of what we've seen before.
And the Barney B-plot is undercooked and kind of gross to begin with. All of his scheming for Robin just makes his pining seem creepy rather than cute. You can't take a serious character (by comparison) like Robin and pair her up with a cartoon character like Barney and have it feel legitimate without bringing Barney down to Earth, and this episode just made him seem like Barney Stinson - exaggerated nymphomaniac. Weak development in a weak plotline.