This mod is absolutely phenomenal, up there in quality with the likes of SMS and Samus. His sprites are a direct translation to the SRB2 style, which makes him fit right in with Sonic and the gang. His movement feels almost 1-1 to an official 3D Mario game, making him easy to pick up and play right from the get go, and, when mastered, his multiple jumps, ability to swim, dive, hammer, crouch/crawl and ground pound, as well as his most powerful ability, grabbing ledges, alongside his powerups allow him to take literally every possible path in SRB2, whether they're Knuckles only, Fang only, or :insert character here: only.
His powerups are all incredibly well made, each offering a unique and fresh take on his gameplay, changing what paths or moves the player would opt to use in most situations. The selection of powerups he has is also incredible, some powerups never being seen in a 3D Mario game or even never being seen again after their debut. My only complaint with powerups is that the ones that shoot projectiles completely remove his ability to use his hammer, replacing the "attack" button outright. Perhaps the mariomenu could have the option to change what button powerup specific moves are used with in a later update?
Despite how incredibly accurate he is to official titles, I do feel like he's missing at least one thing. The ability to pick up certain objects, such as Koopa shells, the balls you stand on to traverse lava, or statues/gargoyles that can be freely pushed around by players. This ability would feel right at home for Mario, as most 3D Mario games allow him to pick up objects and move them around freely. It would also make certain puzzles much more entertaining and less of a hassle to do.
All in all, Super Mario is easily one of the best mods ever made for SRB2, introducing many new mechanics and gameplay possibilities not seen by any other character. If you're for any reason on the ropes about trying him out, your only regret will be not giving him a shot.