Really fun game, feels VERY rewarding to play if you are a fan of self-improvement, like to challenge yourself, like speedrunning/beating your own times until it's no longer possible, etc. etc.
After you complete the game the first time, your best time for completing a level is displayed on screen for the respective level. Great addition, nothing beats spending an hour on Caves-4 to finally hit 15 seconds or discovering you can avoid an entire floor by jumping the right way and seeing yourself shave off an entire second from Sewers-3. Additionally, you unlock the "hearts and clubs" feature, heart being "no deaths on this level", club being "didn't kill any enemies on this level" - one heart and one club being available per level. Very rewarding, fun to achieve, really good feature.
Fantastic game, can't wait for Zak 2.