Zack Zero is a pile of ♥♥♥♥. If you plan to just play through it like a standard platformer (which, to be frank, it is - it's Yet Another Bland Indie Platformer With a Gimmick), don't, because the platforming is in no way good, and the aforementioned gimmick is somehow worse. Zack turns into three alternate forms! Ice, Fire, Earth. Now, IF you want to play it by "its rules", you're going to swap these constantly because higher score comes from raising "combo" by swapping damage types between each attack. The problem is that doing this will chew through your too-small energy meter, so you can basically only get off two or three before forcefully transforming back into Zack Prime or whatever the hell his base form is called.
Let me tell you why the actual mechanics are bad, though: You begin the game as a superpowered wunderkind and then at the end of the first stage, you're knocked on the head and are suddenly Level ♥♥♥, like a later Metroid game. The problem is that ZZ's stages are straight up left-to-right platforming levels, so instead of finding your powers to get cool again, you need collectables. Rareware-level collectables. Each single green dot is some XP! It's less fun than it sounds. The powers, as a result, go from having cool moves and tricks to all suddenly being one-hit-wonders (fire has a hover which you use to clear too-long gaps, ice can slow time to get under too-fast crushers, earth can't jump but HULK SMASHES open rocks beneath you and also is used every time there's a lever because you're too ♥♥♥♥ing weak to pull a lever without rocks on you?? no really).
The levels aren't fun. Period. Only fire has a reliable homing attack (which chews up energy, so you only get 2, maybe 3 off without becoming Zack again) so all the flying enemies are gonna clown on you while you try taking them out, or perhaps the platform beneath you will collapse while you try shooting down the dude who's gonna dive-bomb you mid-leap. I would legitimately argue that Super Mario Bros. (1!) has better platforming and level design than this game, if that tells you anything. Obstacles come in two varieties: "easily ignored" or "KILL AT ALL COSTS", and sometimes it's hard to tell the latter because they look like the not-entirely-3D backgrounds and will come out to wreck you. Oh, also, sometimes the platforms follow the same rule! Have fun telling which random chunk of rock in the caves is supposed to be walk-onable or will drop you to the two-second death FMV for the 500th time. (Yes there is a death cutscene. Dragon's Lair as ♥♥♥♥.)
Don't buy Zack Zero, it's not a good platformer, or a good score-attack game, or really "good" at all. It's complete and utter ♥♥♥♥. If you don't have a controller (I did, for the record!) the keyboard controls are the most spiteful I've ever seen for a game that needs nothing but arrow keys, "switch forms", and action buttons.