Yep, I picked up Super Star Ultra myself yesterday, and I'm liking it lots. I've only played through Spring Breeze, Dyna Blade, and a round of the Gourmet Race so far, but I've already noticed a couple of changes from the original. First, the biggest change (and offender) are the controls. They've remapped the attack/jump buttons from Y/B to B/A - or, you can reciprocate it, and use B/Y. Which sucked for me at first, but I'd gotten used to using B/Y now. Still, the question begs: why? The SNES original had perfectly serviceable controls to begin with.
And then there's the graphics. They are more vibrant, though stylistically, it's been changed up a bit. The backgrounds are looking a little less static than the SNES original, and certainly better layered. Some animations have been changed up, like in the Gourmet Race - instead of Dedede bugging out his eyes when you've won, he stamps the ground repeatedly in anger. And when they've announced the winner, he's no longer peeking out sadly from behind, but has his back turned completely in shame. Personally, I preferred the original in this regard.
Other than that, this appears to be a pound-for-pound perfect adaptation of the original so far. I do wish they've implemented some sort of in-game save or suspend mode for the main quests, though. Playing through each game in one sitting on a console at home is do-able, but may not be as much so on a portable.