Before I even talk about my opinion, what is Double Rainboom? Double Rainboom by a group called Flamingo Rich lead by Zach Rich. Him and his team created the first ever fan made-non-profit episode of MLP:FiM. Before you continue reading this you should watch it first. Trust me.
What did I think of it?
For fan animation, it's amazing. The actual animation was spectacular. The movement of the characters bodies and mouths was almost on par with the show. The voice acting was also great. The voice actors did a good job of sounding like the original voice actors on the show. The jokes on the other hand were hit or miss though. Sometimes the jokes made me laugh, but other times I rolled my eyes. I'm not going to spoil anything, but the twist in the episode blew me away. Did not see THAT coming at all.
Now as a an "episode of MLP:FiM", I would say it was...okay. While I was watching this I felt very uncomfortable. This may be because Flamingo Rich called this a "fan made Episode of the show". This in turn caused me expect the animation, voice acting, and everything else to be like MLP:FiM. It's not.
It's an impressive fan animation, but a sub-par episode.
If you would like to know more about Double Rainboom and the staff of Flamingo Rich, vist their site here.