I gave it a 4/10 and I feel that's being generous.
Barry's characterization is all over the place. He's apparently scared witless about speaking to Iris, even though (from his perspective) he's known her all his life. He's also a grown adult, not a freaking junior high school boy. His parents apparently aren't overly concerned about how proud he is that he asked a girl out, either. He's still got his speed but apparently hasn't used it in the three months we're supposed to believe he's been there, other than to give Thawne hamburgers. He's apparently never heard of this timeline's Flash, nor ever looked up Cisco, nor Caitlin, nor even Wells. Codswallop.
And he apparently doesn't want to be the Flash any more, except the thought that he'll forget he ever was the Flash is enough to make him sacrifice this timeline. And that Wally gets hurt, except not mortally so from the sounds of it. For reasons never explained, Wally doesn't heal, just to up the angst factor I guess.
Caitlin is shoehorned in for no purpose other than to get the actress a credit.
The 'Joe is an alcoholic mess' characterization is never explained and serves no purpose within the story.
Cisco & Star Labs are changed, again, for no real net reason. He doesn't want anything to do with the Flash here, until he does, then he doesn't.
The time travel rules, as usual, make no sense at all, and I'm sick to death of seeing the writers abuse the concept for whatever nonsensical plot they dream up. And then they abuse it again at the end, by concocting this nonsense rift between Iris & Joe at the end, because Barry needs more angst moving forward.
And, of course there's Thawne. The Thawne who became Wells because he murdered Nora and got stuck in the past. Except that Thawne vanished, as have
bothS1 and S2 Barry from that night!
This Thawne goes back, kills Nora -
by the way, that wasn't his original intention when he went there, remember, writers? - then apparently superspeeds Barry back to Joe's house in the 'present'? Didn't he get stuck in the past like he was supposed to?
These writers cannot handle time travel.
Period. Anyone who proposes it as a plot device in any future Flash episode should be fired on the spot.
Really, even if you'd expanded this plot to 2, 3, 4 episodes, it still looks like it would have been a hot mess. Garbage opener. I'd just as soon forget that the ending to S2 and this episode ever happened, except there's the new Iris/Joe rift - with resulting Barry angst! - to waste time dealing with.
And people want to merge this world with Supergirl's? Gah. Keep Supergirl out of this mess, please.
I'm not at all encouraged by this opener. Flash has been going steadily downhill since Zoom was revealed as Jay IMO and maybe there's just no going back.