Very good.
One of the better sprite movies I've seen in a while.
Plenty of great humor, and a good storyline. I didn't know there were so many varieties of thwomps from the whole series. I see a lot of work went into the flash, what with a dozen things going on in nearly every scene at once.
The action scenes were also enjoying. All the way from overtaking the castle, to the Mario and Luigi fight. I did enjoy how each fight involved some crazy combos.
Like I said before, the amount of humor in here was great. I did enjoy the long, drawn-out dramatic music after Bowser is defeated, the whole slow-motion janitor equipment throwing, where it was just a Lakitu hold a fishing pole, and the Kirby cameo appearance. Although there's not as much witty humor as it is slapstick comedy, but that's not really a bad thing.
The voice-overs were great as well. Each thwomp had a voice fitting his/her character. Even Mario and Luigi sounded like........well, Mario and Luigi. Pretty good.
And I also enjoyed the choices of music that were chosen for different scenes. I didn't expect to see the Time Warp at the end. That caught me by surprise, and gave me a worthy chuckle.
Overall, a lot of detail and work went into this flash movie, and it's easy to see all of that. So kudos on such a great flash. A well-deserved 10.