What an absolutely, utterly excellent movie. I'm sure you could probably pick apart the relatively simplistic love story, or argue that the characters could be a little more developed, or even consider how the central blackout mechanism makes no sense... but why would you? Instead, you could simply get lost in the charms of the wonderful performances, excellent scriptwriting and genuinely entertaining, original plot - and that's not even mentioning the (obviously) exceptional soundtrack!
After a string of big-budget action films that deliver exactly what you expect (not a jab!) and marginally-to-completely disappointing summer flicks (definitely a jab!), going to the cinema to see something that just gently drew me in, made me laugh, made me care, and gave me an overall great time was a real delight. It even managed to introduce me to a couple of Beatles tracks I've been happily listening to since!
There's genuinely very little I want to say about Yesterday, mainly because I just thought it was wonderful. Everything about it is just charming, even Ed Sheeran is interesting (enough) to watch, and every now and then it has these perfect moments that are narrative gems. The Pepsi reveal was perfect; the subtle changing of the headline on the newspaper cutout in Jack's bedroom in a world without Oasis, plus all of the Google page easter eggs, are just clever; that big twist cameo which Robert Carlyle nailed completely; even the way that one major subplot just fizzles out (intentionally) still brought a smile to my lips*.
In short: go and watch this movie. Go in expecting a fun ride and you'll come out beaming from ear-to-ear, humming away like a madman. Definitely a feel-good triumph, absolutely a blast, and worth every penny.
* Even if, as Alison pointed out, it seems a little unusual that they would be able to buy a yellow submarine toy that perfect. I mean, why would it even have been made!