I will say first my experience is primarily with MBG (and MBU years later, which I think might be my favorite iteration of MarbleBlast in general). I also tried a little bit of the original Platinum back in the day, but not a whole lot.
But as far as PlatinumQuest goes? I think this was quite an enjoyable experience, that has a ton of ambition. My primary issue with the OG Platinum was that it felt a bit too tough as nails for me; it was clearly geared towards masters of the original game, and I just didn't have the skill level or patience to see the whole thing through, especially with its use of engine quirks that were mandatory for finishing some levels (whereas in Gold they were just optional speedrunning strats). But in PQ it feels like an earnest effort was made to make the game feel more accessible to a wider range of players, which instantly sets it apart from its predecessor.
With all said and done I enjoyed PQ a lot. The soft, pastel-ly colors to the textures are very pleasing to the eyes, but still manages to make the environments very colorful. Everything about the look of this game is pretty much a direct upgrade to what came before. Even the smaller things, like the addition of architecture and nature-like objects makes the levels feel more lively. I really enjoy the grass and flowers, the cartoonishly round trees feel right in place, and even all the thin wooden and glass-like structures give it a very cozy feel to it. I also really like the marble selection. It's very rare that I like the default marble skin, but that deep indigo marble with the soft cloudy patterns I found to be really well-designed and I kept it for my whole playthrough.
Now the main meat of the game, the levels, I found to be really enjoyable for the most part. Although I could do without some of the "revisited" levels (which sort of makes me feel like there may have been a struggle to fill the level groups with enough content), they weren't too frequent and overall the selection was quite varied. I think some personal stand outs for me were all the construction levels, the serendipity garden level is just stunning to look at and explore through as well. I also really enjoyed the playground level too, though personally the par time felt a bit too strict to really enjoy exploring the architecture in, it might have worked better just as a level where you collect all the gems and then head to a static finish pad. Speaking of the gem levels, I really have to commend the concept for some levels where you only have to collect a certain amount out of the overall total available, but you can optionally try to get every single one to "ace" the level. Honestly the amount of gimmicks present in the levels really feel nice and mix things up in a way where I probably would have gotten burnt out in the original Platinum.
Now, while I did enjoy my time with PQ, I do need to get a bit salty, because I have a plate of beef that I need to address. I mentioned this in one of my support threads, but the graphics on my Mac were a little blurry and gritty. This is likely due to Macs these days utilizing their patented retina displays so I can't really blame the development team for this, but it did still bother me a little because I bet this game would have looked even nicer with a clean screen resolution. (I may try testing something with Whisky/Wine in regards to this).
I will also add that not all levels were winners. I must address first that when I played MBG I preferred using WASD for the camera movement and the arrow keys for the marble movement, as I just wasn't a fan of using the mouse for this kind of game. With that said, I had a great deal of difficulty trying to complete some levels with my preferred key setup. There were a couple levels that were pretty much impossible for me to beat because it expected the precise movement of the mouse to keep up with all the action.
I also did not like the levels where you're forced to stay above a certain speed. Because of how unstable the speed build up is in this engine I always play more cautiously so I don't end up flinging myself off the level, but in these levels I'm basically being punished for going fast, but not crazy fast. "Momentum" (and its revisit in the expert group) is the biggest offender for me, due to its rather unfair obstacles that will kill your speed instantly if you so much as brush against them. Honestly I think these levels would be better if the whole speed minimum gimmick was just scrapped. These levels already encourage you to go fast, so this just felt like an unneeded level of frustration.
I was also not a fan of the 2D levels. Although I think its an ambitious gimmick, its mainly marred by not being able to see out far enough, which causes some levels to invoke the "blind leap of faith" trope, which is not very fun in my opinion. If the camera could pan out more (or inversely, shrink the size of the levels), it would be a lot more manageable.
There's also a couple other misc. things that bugged me. Such as the bonus unlock system, which I honestly didn't really like. I think it would be better if the bonus levels had more general requirements (such as "beat all intermediate levels," etc.) or maybe if the category itself unlocked after you beat "Manic Bounce" and reached the end credits, and you could just play everything inside it afterwords. I was also bugged by the online mode's custom levels tab because it was so tedious to search through that massive list of levels. It also bothered me that there was a dropdown list for "installed" custom levels, but the list kept clearing every time I closed the game (so they weren't actually installed). It would be so much better if you could, say, hit a download button somewhere, which would automatically download the level to your local missions folder, or if there was some kind of heart button that would bookmark the levels you enjoyed for easy access (it could replace the "installed" tab in the dropdown menu).
And I think that about wraps up my thoughts? I'm very sorry if this was quite a bit to read, but I had a lot to share in regards to PQ. Overall I found it to be a very enjoyable game. Plus with it having the level archives from previous games (as well as keeping their unique sounds and graphics for maximum authenticity), I think this could be the definitive way to enjoy MarbleBlast in the modern era. It just has a few bumps (as mentioned in the above paragraphs) that keep it from being a perfect experience.