To begin with, it's hands down one of the best episode of the show so far, and it's a nice enough show. Thanks to Greg Berlanti and Andrew Kreisberg (the two having previously created Arrow and The Flash), DC's TV universe is not a disaster like the movies. Flash who can travel through time & parallel universes, accidentally lands up in National City. And the fun begins!
Like the crossover with Arrows, this one is quite fun. The chemistry between Grant and his ex co-star from Glee, is amazing. Which makes the chemistry between Flash & Supergirl fun. Both are funny, nerdy, cheesy and awesome! They team up together against the duo of Livewire & Silver Banshee, and they all fight like women (as Flash says!). Flash helps Supergirl save the day (spoiler: although they don't actually save the day, it gets saved by Firefighters. Kinda boring, but...). And Supergirl helps Flash get back to his Earth. And in between, there are races/competitions (guess who wins?) and lot of funny jabs.
Both Grant and Melissa are phenomenal. Seeing this James Olsen jealous, was nice for a change. Winn as the mega nerd of Supergirl, is cool, as usual. But unfortunately, he lost his new found lady love, to Silver Banshee (or who became her). Rest of the cast is amazing as usual. The episode's teleplay was written by Andrew Kreisberg & Michael Grassi, and directed by Nick Gomez. All in all, this was an amazing episode, and hopefully we get to see more of these.
Fun Facts: This marked the first episode of the series to feature a character from the DC Universe that appears on another show, and the first character from a CW series to cross over to a CBS series. The episode title was inspired by the World's Finest Comics series, in which Superman would team up with various other DC superheroes, including the Flash.