I never saw everyone's problem with SSE and thought it was okay and mildly fun, but Project M 3.5 turned it from "meh/okay" to "extremely fun/holy crap, this is SO much better!" in one fell swoop. It's like the original SSE only you can move as freely as you could in Melee's Adventure mode and you actually stand a chance against the annoying and often cheap as a hell enemies because your characters are stronger and faster this time around. Now things still get kinda hilarious once you run into Duon and the "Not Gutsman" bosses since they have Roy and Mewtwo's sound clips placed over them respectively, but it's nothing that killed the experience. If anything, it just makes it more amusing. Even The Great Maze was no longer a dragfest of a hassle anymore due to how quickly you could move around and kill the emerald copies and all of the bosses before reaching Tabuu. Clearing SSE in a whole day on Hard is very much possible now. Though honestly, for the sake of the enemies being balanced and not getting so frustrated I don't think SSE should be played on anything above Normal or Easy.
So yeah, Brawl's SSE was "meh", but I LOVE PM's SSE so much. It and Smash Run are so much fun for me to play. Smash Run is pretty much fun for me until it's time for the final game at the end unless it's the "Run!" race game.