Flash 3 x 08 "Invasion!" spoilers

Well, the second part(though, really the first part) of the CW's crossover was pretty good. Kara was brought in about fifteen minutes into the episode, reusing the scene from last night's Supergirl where Barry Allen and Cisco Ramon arrive on Kara's Earth(Earth-38 according to Cisco's reckoning). Kara had a cute scene where she met the rest of the heroes for the first time. Nice moment between her, Barry, and Oliver Queen, where Barry touched upon how Oliver had trained him in the beginning(including Oliver mentioning how he shot Barry). And her interaction with Mick Rory aka Heat-Wave was pretty funny.
I liked how it was quickly established that she's the powerhouse in this massive team up, kicking everyone's ass in a training session. There was drama involving the existing plotlines on Flash, Arrow, and Legends, including everyone(except Kara) having trouble trusting Barry when he admitted to screwing up the timeline between seasons. Barry's supporting cast definitely took center stage tonight, with the continuing story of Wally West coming into his new powers.
The fight scene between Barry and Oliver against, well, everyone else(mind control) was pretty well done, and I liked how Barry was forced to trick Kara into destroying the mind control device, since there's no way in hell he could take her in a straight fight(though it turns out he can phase through Kryptonians just as easily as any other solid matter).
All told, I thought it was a good way to integrate Kara into this Earth(even temporarily), and I am looking forward to tomorrow night's episode where she and Barry work with Oliver's new team. Hopefully it'll give her some perspective, given her dislike of vigilantes.

"Well, the second part(though, really the first part)"
Well said.

Part one on Supergirl was a complete tease and had nothing to do with this event.
However Supergirl was Super in this episode and it really highlighted just how powerful she is.

Part 3(really 2) was on this evening. Now, Kara got only about ten minutes(if that) of screen time, she and Flash helping the junior members of Team Arrow take on a one off cyborg villain who had a piece of tech needed to help track the abducted heroes. While the episode was part of a crossover, it was also Arrow's 100th episode, so the focus on the show's regulars was more than justified. As a fan of Arrow from day one, I gotta say that it was very well done. We got the return of Katie Cassidy as Laurel Lance, as well as Jamey Sheridan and Susanna Thompson as Oliver's late parents.

The plot was Oliver, Thea, Sara, John, and Ray finding themselves in a virtual reality, where(among other things) Oliver had never lost his parents, nor had he become the Green Arrow. Instead, he was about to get married to
Laurel and his family was alive and happy. Not too dissimilar to Kara's dream of being back on Krypton last season. Of course, they all start having flashes of their real lives, realizing that it's just an illusion. Fantastic acting on the parts of the cast, and the pain of having to give up the happy illusion was tangible. The goodbyes between Oliver and Laurel, Robert, and Moira were especially heartbreaking. And the shot of him seeing his parents, Laurel, Tommy, Roy Harper, and Felicity(accompanied by audio of past encouraging words from them) right before leaving the simulation was a nice tribute to years past.

There was a nice meta-joke, where the character of Tommy Merlyn was mentioned as being in Chicago, working as a doctor. Colin Donnell, the actor who played Tommy in Arrow's first season currently stars on Chicago Med.

Honestly, the only real disappointments to the episode were Supergirl's limited role(which, as I said, can be excused given that the episode was supposed to be a love letter to Arrow's fans), and the fact that they couldn't or wouldn't get Manu Bennett to return as Slade Wilson/Deathstroke, instead relying on a stuntman in full costume.

I may be a little biased, given that Arrow has always been(for the most part), my favorite series in the CW's DC-inspired shows, but I thought it was a very good episode.

Tomorrow night is the final part of the crossover in Legends of Tomorrow and it's looking to be a classic huge superheros vs aliens brawl. Ought to be good.

The final part of the crossover was pretty good. In terms of Kara, she did get more screen time than she did last night, though again, the focus was on the regular cast of the actual show, in this case, Legends. Ongoing subplots were touched on, and there was some resolution to the tension between Barry and Cisco. Also, it was revealed that the reason the Dominators chose to attack Earth was because of Barry's screwing with the timeline. Like in the comics, they perceived the existence of metahumans as a threat. And like in the comics, their big plan was to use a bomb designed to target metas.

There was some tension between Kara and Oliver, mainly due to his desire to keep her at arm's length because he wanted to maintain some sense of normalcy in his life. That though was resolved when, during the fight, Oliver was knocked off the roof, Kara catching him. The final fight was well done, the effects being what you'd expect from a TV budget. Ultimately, it was Supergirl and Flash tagging the Dominators with devices designed to cause them extreme pain, as well as Firestorm transmuting the meta-bomb into water that forced a retreat.

There was the obligatory party scene at the end, where Kara mentioned talking to the President about forming an organization like the DEO on the Arrow/Flash/Legends Earth. Kara, Barry, and Oliver had a nice moment(including a group hug initiated by Barry), that seemed fitting given that they're the CWverse equivalent of the Trinity. They're the anchors that tie everything together, Oliver and Barry especially.

Before returning home, Kara received a device from Cisco that will allow her to communicate with Earth-1, as well as open breaches from her Earth. So that leaves the door open for more guest spots/crossovers down the line, including the musical crossover with the Flash in the second half of the season.

The episode ended with Barry and Oliver in a bar, commiserating about how weird their lives had become, and how they wouldn't have it any other way. While the crossover was by no means perfect, it was a nice reflection on how much this TV universe has changed since Arrow first premiered. It started out as a street level crime drama set very much in reality and nothing really fantastical about it. Oliver was, for all intents and purposes, a serial killer at the start. Now five years later, it's evolved into a world where superheros, metahumans, time travelers, and now aliens are commonplace.

No word on when the next new episode of Supergirl will air(beyond the title "Supergirl Lives" and that it's going to be directed by Kevin Smith), though Flash, Arrow, and Legends have their mid-season finales next week. Hazard a guess though, but I'm thinking "Medusa" was the mid-season finale. Guess we'll have to wait and see.

Personally I think the best 1 liner for the crossover was done on Legends.

"She looks a lot like my cousin." Had to LOL at that one.

IMO. The end fight would not have happened. The Flash would have been gone before Oliver said We won't let you give up your life. Supergirl would not been able to stop the Flash into. They are about the same speed, and he would have had a lead. But that would have ended The Flash as a TV show, and we don't want that.