The Flash/Arrow crossover episode has them battling Vandal Savage in a two part extravaganza as Vandal Savage tries to run his record in battles to the death against Hawkman and Hawkgirl to 207-0. Below are the highlights of the battle.
***Spoilers Ahead***
Positives:

---After last year's sort of lackluster crossover with them having episodes where they met not really related, we get a battle with Vandal Savage, one of the DC Universe's best villains and he's well realized here as an immortal full of arrogance and quite deadly. He's a great menace and ultimately he makes this an event rather than an obligatory gimmick.

---Oliver Queen has some great moments and really works well off Barry with his annoyance at having a woman who Savage wants killed dropped into his lap with no clues about her and resistance to it. One great moment was in the second part when he learned that Barry had time traveled back in time and he decisively moved in favor of correcting the timeline without the hemming and hawing Barry would probably have probably on his own. I also like that Ollie questioned the idea that the timeline would really care whether he told his girlfriend about his son. Finally someone questioning the personification of natural forces like "time" and "the universe" by cast members in the Flash.

---A great story for Cisco as he struggles with being supportive while also slowly losing his girlfriend to her "Soul Mate." for four millennia where they fought evil (in between being beaten by Vandal Savage.) There's a great deal of sweetness and protectiveness in the way that Cisco is portrayed that really makes the story work.

---Shorter first scene of, "Legends of Yesterday": "Yeah, we're ripping off Avengers: Age of Ultron. You want to make something of it?

---Finally, some acknowledgment that trying to work out personal issues while trying to save the Universe isn't always a good idea.

--While I'm no fan of the reincarnation plot with Hawkman and Hawkgirl, it is something borrowed from the DC Universe and it's true to the characters golden age version. At lease they didn't trying to combine it with the Silver Age Alien police story from the Silver Age like Justice League: Unlimited did.

The Negatives:

---If you're not following Arrow (as I'm not), then much of the first half of The Flash seems like you're attending a friend's high school reunion. A lot of- characters (particularly from Arrow) are just there. Maybe next year, a smaller scale crossover with the Flash going to meet Ollie or vice versa. When two team-based shows meet, it just gets overwhelmed with everyone catching up with everyone else and meeting the new guys and the new headquarters.

---I know John Barrowman best as Captain Jack Harkness but in The Flash, he gets to be Captain Exposition as his character Malcolm Merlyn gets to show up to do an info dump on Vandal Savage's history. That was the best way they could think of to do that?

---One should not try to do Ancient Egypt on a budget.

---There's so much wrong with the DNA test in Part Two. First, just because you're the fastest man alive doesn't mean you can do DNA tests at the speed of light. The worst way to hide something is to try and hide it at superspeed because it makes a sonic boom. Finally, the idea that Felicity recognizes Oliver's DNA pattern on sight had my wife and I both laughing at the absurdity of it.

---The way the fight between Felicity and Ollie plays out is over the top and the "I recognize your DNA" makes it seem contrived. I think the way the whole set up of the long lost son is handled was melodramatic as well with the mother unreasonable and Ollie unwilling to even try and counteroffer to just tell his girlfriend and no one else even apparently knowing the consequence of keeping this from Felicia.

---Hawkman is another character from the upcoming Legends of Tomorrow who doesn't have a particularly strong introduction and this may not bode well for Legends of Tomorrow.

Overall: A fun, but flawed romp with Vandal Savage, flying heroes, and some truly fun moments between the Flash and Barry. It's definitely a step up from last season's crossover. Still, bigger isn't always better and there were lots of characters getting lost in the shuffle. Rating: 7/10