Its basically a port of the NES version. I haven't gotten far enough yet but I assume that Extra Mode will again be No-Save. I'm not terribly sure since its been a while since I played the NES version but the game looks slightly more colorful than it originally did. I can confirm that those Rotating tower segments in Butter Building are intact, and even better looking than before thanks to not having that ugly pallete [tornado fang] up at the edge of the screen.
The only thing it doesn't have is the little "How to draw Kirby" intro. EDIT: Its there, you just have to wait a bit for it to show up.
EDIT 2: Oh yes, I should mention that most of the PS2 Classics available are pretty much "eh", they do have both God Hand and Odin Sphere available, so for the folks who either missed those games the first time around (Like me) or want to play them on the PS3 without having to dig the PS2 out, this is the best opportunity to get them at a decent $10 a pop.
EDIT 3: Because I can't go without updating. Added Namco Museum Essentials (PS3) and Metroid 2 (3DS)