I 100% completed this game without save states and I have to say it: this is a very impressive rom hack, especially for a hack that came out in 2014.
Super Mario Bros. 3Mix is one of the best 2D Mario offerings currently available. If you think the New Super Mario Bros. games are too easy, or prefer the physics of older Mario platformers like I do, I highly recommend this game. While it has a couple of bad levels (the only one that really comes to mind is 8-2), the vast majority of the game is well designed, and constantly introduces cool and fun mechanics.
Coming from the perspective of 100% completion, Super Mario Bros 3Mix can be either as short or as long as you want. Most levels are not very long, but the games longevity is significantly increased by the return of a lovely collectable from the New Super Mario Bros. series: Star Coins. Hidden throughout every level are three Star Coins that you have to go out of your way to find. This means you'll be spending most of your time thoroughly combing every level for them if you want to see everything this game has to offer.
(SPOILER ALERT, SKIP THIS PARAGRAPH IF YOU CARE ABOUT THAT) If you collect every Star Coin and beat the game, you are presented with one final challenge: various Comet Challenges have been scattered across every world, and you must complete all of them to unlock the final world. The final world is a fantastic send-off to the game, with much appreciated references to Super Paper Mario. It's a fantastic reward for getting every other collectable, and I personally thank Southbird for having a better 100% reward than most actual Mario games.
The game has a lot of interesting ideas, and tackles them with gumption. Being a very ambitious rom hack, however, leaves many areas of the game unpolished. The most noticable one is the music; the main melody of many songs get constantly cut out by the sound effect for jumping or collecting coins. While I understand this is a limitation of the NES, putting the melodies on sound channels that aren't the jump and coin ones could have fixed this ever-present issue. The game's collision also frequently breaks with enemies clipping through walls, floors, and ceilings. This seems even more frequently if the games framerate drops, which is something that happens with frustrating frequency.
Although this game can be frustrating, it's hard to stay mad at it too long because of how good many of the levels are. Almost every new idea the game has is well executed and fun, and even if it didn't rock my world the entire time, I still recommend the game to those looking for a new 2D Mario game that isn't too easy, but doesn't bust your balls.