My feelings are... mixed. I have some great things, some mediocre things, and one especially unhappy thing to say about this game.
First off, I seriously admire how much detail there is. From beginning to end, the artwork is solid and beautiful to look at, and nothing ever feels out of place. The parallax-scrolling title logo in the first stage was a really cool way to introduce the game, and the controls are easy enough to figure out by the time you get to Level 2. Everything runs very smoothly, and you certainly caught the feel of old 16-bit classics like Sonic the Hedgehog with the catchy music and level design--except here, the controls are more manageable and the fast pace of the gameplay is very infrequently interrupted. The difficulty curve is also pretty respectable, and I can tell the game is just designed to feel rather satisfying to play through.
Sometimes it is difficult to distinguish between the background and foreground, or which platforms you can jump through and which ones you can't, which can be pretty disconcerting at times. Occasionally, I found it unclear which direction I was supposed to be going to reach the end of a stage, starting with the first maze-like "Oxygen Plants" level. And when I finally met the requirements to enter one of the bonus stages to save one of the other Ripple subjects, I found the penalty for not collecting quite enough vials within the time limit to be really unforgiving, so completing the game 100% sounds like it would be more frustrating than fun.
I also found it irritating that the protagonist has a maximum health meter of four hearts most of the time, but he always starts out with two hearts upon entering a level or respawning, and if he gets hit even once, all you hear until you either die or find another heart is *blip-blip-blip-blip*... From what I've been able to tell, there is no way to turn this alert sound off without turning off all the sound in the game, and this saddens me because I enjoy the music and sound effects.
Now, for my biggest complaint, and the main reason I have mixed feelings about this game: If I'm being completely honest here, even with all of its beauty, I feel like this is almost as generic of a hop-and-bop game as they come. From a gameplay perspective, there's not much about this experience that makes it stand out. You run, jump, and swing a beam-sword, and meanwhile there are a bunch of small objects to collect. I was intrigued when that hover suit and boomerang disc were introduced, only to be disappointed when I found how rarely they were utilized throughout the game. As I played on, I kept hoping for more unique mechanics--new ideas that built upon past levels to create new experiences. Instead, every level felt basically the same, just laid out a little differently, save for the boss battle and mini-chase sequence at the end.
If I had to summarize my thoughts, I'd say Ripple Dot Zero does everything that it does very well, but some areas could have been improved, and the overall experience had a depressing amount of missed potential.