Okay, so when I first started watching this video, I had NO idea what to expect. I'd never seen the original series, so I don't even know what inspired me to click this. I figured, "Hey, it got on the front page, and it's going to be a series, so why the heck not?"
And then I saw the Sonic characters. I was like, "What the hell?! Is this some sort of crossover fanfiction? Why would Sonic characters even be IN a Mario series?" That kind of annoyed me a little, but I decided to keep watching anyway, because the animation was absolutely beautiful.
When the episode actually started, I was a bit disappointed that there wasn't any voice acting. I realize that would have made this an even bigger project, and it would have taken even longer, so I can forgive the lack of voices. I still think having voice acting would be pretty cool, but I'm willing to put up with reading text for now.
As the episode went on, I realized what an absolute gem this was. The animation contains probably the best sprite work I've ever seen, with smooth movements and real nice cinematography. Also, I found the silly slapstick humor to be quite enjoyable, and I even got a laugh out of it occasionally. The action sequences were brilliantly put together, and each action flowed nicely into each other. I'm not normally a fan of long fight scenes, but I couldn't stop watching because of the awesome animation, plus I really wanted to see what would happen next.
I thought that each character was written really well; everyone acts the way I'd expect them to. At first I thought Luigi was way too cowardly, but then when he tried to sacrifice himself for Mario, I actually felt SAD. And I don't normally cry over stuff like that! Maybe it was the music? By the way, the music was also really nice. I have to give you props for using Undertale music XD
All in all, this was WAY better than I thought it was going to be, and I'm so glad I watched this. I can't wait for the next episode-- heck, I might as well watch the original now. You've officially impressed me.