Okay, here's my opinion, off the bat:
Basically, it's a great hack, but it's just a little too easy and really, really short from what I can see. Okay, maybe that's because I'm good at Mario hacks, but I didn't really find anything challenging outside the castle levels and certain boss battles, and the game had maybe, four secret exits across the whole game (one switch palace, two secret side rooms, and one shortcut to a later level, along with one I forgot which is required for progress Choco Island 3 style.
The bosses and ASM are good, although it froze after one of the boss battles for me, and I had to skip a certain level with Game Genie codes. The first boss was awesome, being a really cool ghost boss with teleportation and such like, second was just quite boring and too easy, the Chomp Shark chase was clever... and then the three bosses were easy to find sprites by Sonikku, which made them quite... well, underwhelming to battle (Balrog, Toroko, Night Spirit). You seemed to use a lot from Brutal Mario after that, with Xanbie (the weird fire bird head thing from an early castle) and a boss from Mega Man in some of the later levels. The final boss, a weird wizard looking guy was okay, but I thought I was gonna get to fight that Yoshi's Island Bowser you showed off a long time back.
Fortress wise, it was a bit dull. Okay, one boss was Boom Boom (carol's version I think, which is weird, because you do have the ASM skill to make your own equivalent, which would be more impressive), another was a ground pound Koopa, another a Blargg boss and finally, a Big Boo boss in the chapel level. Nothing remarkable, indeed, apart from Boom Boom, they're all just standard stuff from the sprites section with basic rooms and attacks.
Level design wise, it was excellent fun, with a new twist in every level. However, it was a bit too linear, and it never really got very difficult at any point. Another two worlds would have done great to be honest, especially with a final level count of less than 60.
The overworld was great, with graphics from various games (I remember Minish Cap graphics in one section). However, it passed a bit quick, and you never really get to explore enough of it.
Graphics were mind blowing. Every world (and more precisely, every castle, fort and ghost house) had new, ultra detailed graphics taken from the likes of New Super Mario Bros, Yoshi's Island DS and all manner of RPGs. Personally, I'd love to use one of the backgrounds, they all looked fantastic.
Music was cool throughout. Can't say much more, it was just great and without any issues.
Oh, and I see you took a lot from VIP 5. Okay, I'm a hypocrite, I use a fair amount of the stuff myself, but with the Zingers, Squawks sprite and on/off invisibility gimmick, it all looked a bit like I'd seen it before.
Would I recommend this hack to people? Yes, it's fantastic. It'd be great for a Youtube Let's Play series to be honest, if someone like raocow was interested. But it's too short and linear, and took me about an hour to complete in total.
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As for Mario's Nightmare Quest? Well, it's currently on Fusion Gameworks, ROM Hacking.net or the GCN at the moment.