I loved it. I would have 5'd for any of the games on their own, everything was really fun. Even though SFB are my favourite artists, I liked their part the best.
However, for the first three level I found new guns more of a nuisance than a help, because they switched my gun from a sawn-off shotgun to the new weapon. Theres really no point in other weapons than the shotgun. In the madness levels it caused me to die a few times.
The songs were great, especially in the Super Flash Bros part. One problem with the sound was that it dragged on into the Flash Bros. Death video, and I couldn't hear the dialogue.