I'm definitely going to get bored of spirit battles quickly. It is the most repetitive mode ever, I don't need to play it to say that. I saw a lot more similar fights than I expected; lots of, "basic 1 vs 2," battles and definitely waaaaaay too many in the category of, "Battlefield/Omega form stage with 1 different thing going on." I recall past Event Modes having a bit more variety to their challenges.
Most of the special rules were really insignificant. Like, what's the point of a spirit battle that's just... a character alongside an assist trophy, or holding an item, or using a move .01% more often than the rest of his/her moves, or spamming a move that makes them vulnerable and easier to kill, or literally just having slightly more power/speed? You can instantly negate any of the basic hazards like floor and wind with a single support spirit, so what's even the point of having challenges like that? I plan to stick to Special Smash if I want a spiced up battle.
I don't think feeding and leveling is going to be fun or rewarding enough for me to bother with it. Eh, just feels like if I was struggling to beat a spirit battle, the multiple characters at my disposal with their own sets of strengths and weaknesses seems superior to some lame grind-for-a-minor-stat-boost option.
The skill tree however is definitely something I'll use. It's much more simple and to the point; get the skill spheres, get the boost. If you're going to include any sort of customization, that's as much as you'd need in a game like Smash.
Overall I don't think it's going to be a very fun mode, it's a 3/10 if I were rating it.
But like
THE DONKEY KONG COUNTRY MAP!
THE LITTLE ZELDA PUZZLES!
YO, A PLAYABLE MASTER HAND SEGMENT!
OH JEEESUS THAT FINAL BATTLE WITH GALEEM AND DARKHON!
There's so many little things here and there that are so amazing, that just knock it out of the park! They should have done more of that and less of that other crap. I wish I didn't have to trudge through the garbage main mode to get to the really cool stuff.