Here's my thoughts:
Overall, I thought the Invasion crossover was so-so. We were told it would blow our minds and that it would have special effects of the equivalent of a Hollywood movie, but I felt Invasion came up short. It was nothing like the comics and we only really got to see just a few minutes of footage vs The Dominators. I get it, it's television and the budget is a lot restrictive, but the story was a let down as well.
For one, Felicity is a terrible character and destroys every scene she is in. I'm not saying Emily Bett Rickards is a bad actress, I'm saying the writing is down right awful. Ditto for "Team Arrow," as Wild Dog, Ragman and Mr. Terrific were just as bad as Felicity. It's no wonder Stephen Amell is questioning if they will be doing another season: The "General Hospital" stuff on Arrow needs to go desperately and ASAP. Stephen Amell's Oliver Queen BA Arrow doesn't fit with this goofy and corny dialogue in the least bit. Honestly, they should make it like a Christopher Nolan movie in terms of tone and get rid of all the "General Hospital" garbage. The show was much better when it was just Ollie and Diggle (more Diggle is a good thing). I also thought it was awful the way they got Supergirl out of the picture with Oliver Queen telling her he needs things "real" right now or whatever. That was pretty lame and was an obvious excuse to either get the uber powerful Supergirl out of the equation (because writing powerful characters is "difficult") or lessen the screen time of Melissa Benoist due to contract payments, etc.
The Flash stuff wasn't much better as there was way too much drama between Barry and Cisco. That could have been dealt with in the main series, and even better, Cisco could have tried sending his brother or himself a note to the future (a la Doctor Who "Blink" or Back To The Future) to stop his brother's death, which would have been a much better moment than his Eureka! "I just changed time" excuse and "now I can be friends with Barry" moment. This season of The Flash has been a let down as well, which is also to be blamed on all the "General Hospital" soap opera drama going on between characters.
The best part of the Invasion crossover, and it's been the best of all the DC CW shows this year, is the Legends of Tomorrow. Great story. Great drama. No girlfriend/boyfriend BS. Cool sci-fi stuff happening. Heatwave is the best character bar none. One big thing they missed, however, was an opportunity to have the Legends chat with Barry about the Speedster that has been plaguing them this season. Why wouldn't they question Barry about it, who is a speedster himself? I think there may have been something briefly touched upon, but then at the exact same time it was blown over. Obviously, they would both put two and two together and realize Reverse Flash is behind the time aberrations on this season's Legends of Tomorrow because Barry let him loose, which would have made Barry even more guilty (etc. etc.) Also, I was lost when Professor Stein's daughter showed up on the Waverider. I thought Stein left her at the house when they first met because, you know, that was the first time they had met? Then the next thing you know, she is right next to him on board the ship? Huh, did I miss something? Another thing that bugged me was when Citizen Steel got captured in that he was stuck with a needle. Umm... He can turn his skin to steel, right?
Regrading The Dominators, at least we are going to see more of them in Supergirl, but they were nothing like the Invasion comic, which saw The Dominators as the King of the chessboard as they recruited other alien species to do the actual warring with Earth. I don't even believe the Dominators set foot on the Earth in the comic, which brings me to another missed opportunity with the Daxamites. Obviously the Daxamites exist, as Mon-El is a part of Supergirl. They, too, were a big part of the Invasion comic and had nothing to do with this crossover. A second missed opportunity in regards to Supergirl - and I know it's not The CW/producer's fault as DC only allowed two episodes with the character - but Superman was a major part of the Invasion crossover and was missed here. Supergirl could have filled his role, but the story was way too generic to have done her any justice.
My thoughts on Supergirl in Invasion? There aren't many. Her own "Invasion" episode was really weak as it only had about 30-seconds of footage. To be truthful, I remember reading an interview with one of the producer's ahead of time who basically "warned" us about the fact, so it wasn't really that big of a deal to me, but I know it was to some viewers. You should't have to warn viewers ahead of time. The episode probably shouldn't have been promoted as being an Invasion episode, which turned out to be nothing but a cheap gimmick.
I could also do without the politically correct BS that's been going on with these shows as it all feels really, really forced -- again more bad writing.
The good? What we did see of the DC Justice League vs The Dominators was pretty cool. I was happy with the look of the Dominators. I was happy with the look of the inside and outside of their spaceships. The 1950s footage was super cool (probably the best scenes of the entire crossover). The Hall of Justice Easter Egg was damn right cool. Brandon Routh's Easter Egg was a welcome touch. The Future Flash message was awesome.
So for my final thoughts, I'm almost wondering just how much footage was cut from the Invasion crossover? Stephen Amell did say they filmed five episodes, but in reality we only got about three. Hopefully next year they can ditch and limit the drama and get straight to the action. It's comics, guys! And make the story better (If it's not Invasion! don't call it Invasion!)!