(Spoiler Alert!)
Double Rainboom is a very cool crossover between PPG and MLP. I love the way they animated the PowerPuff girls (nostalgia!).
However, I feel like the creators could have been more original when making the film (not using so many, or if any, pop culture references), and there were lots of filler moments throughout the plot.
Another thing to point out is, even though, of course, the monster was annoying and destructive, it wasn't nice to see Rainbow Dash roll her eyes at him like that (especially since it was right after he apologized; watching that might teach kids to roll their eyes (they may not understand that it isn't a nice thing to do; that is another filler to be taken out)).
The ending was good too, but I also think that Twilight could've at least said "Rainbow Dash! You're alright!" instead of just glaring at her, because Rainbow could have been permanently trapped in the other world (I understand that Twilight was angry about Rainbow stealing her potion, but knowing that what Rainbow did was risky, they could have never known if she was coming back, so Twilight should've expressed her relief; Rainbow Dash could've never made it back).
Overall, it was a cool idea, and it was very fun to watch!