I never owned this as a kid, but my friend did so I played it at his house. Thus, I've now beaten it for the first time as an adult. For Mario's first foray into 3D, they shockingly got it pretty much perfect on the first try. In fact the game's formula proved so popular that it would be copied by other 3D platformers like Banjo-Kazooie and Vexx
The game takes place in Princess Peach's castle which serves as a hub world linking all the levels together. Each level is found somewhere in the castle in the form of a painting - Bowser has stolen the magical "Power Stars" and used their power to curse the paintings. This "magical painting" conceit explains why the castle links to all these different worlds, as well as kind of explaining why they are such abstract spaces such as a small island floating in space (Whomp's Fortress). Each level has 6 total stars to collect (plus a secret 7th star that appears when you collect 100 coins), but the genius of the game's design is that you don't HAVE to collect all the stars to finish it, and you don't even need to visit all 15 worlds if you don't want to. You only need a certain number to unlock new areas of the castle. Therefore, a beginner can skip over stars they find difficult and collect the easier ones, while skilled players who want to challenge themselves can go for all 120 stars (there are some extras hidden in the castle aside from the 6 per stage). Also, the stars don't have to be collected in order. Each stage also has 8 red coins, which will always be one of the stars. The first star is usually something like "Get to the top of the level" which gives you a good idea of the level's basic layout. Then I usually saved the "100 coins" star for last when I was familiar with the level and could figure out where to get the coins. Although, the 100-coin star can be collected at any time. Normally, collecting a star will boot you out of the level back to the hub world, but the 100-coin star does not, and appears wherever you collected the 100th coin. So you can get it while going for another star and get two at once.
When I started playing, I didn't think I was going to go for 100% completion, but I ended up having so much fun with the game that I decided to fully complete it. That said, I probably wouldn't have if I was playing on original hardware and didn't have access to save states to make things less frustrating. One thing I noticed is that the first enemy you face is one you have to get behind, pick up and throw, which teaches you how to defeat Bowser as he's fought the same way. Actually a lot of enemies in this game require circling around them, perhaps to show off the novelty of it being in 3D, as it was a launch title for the N64 and therefore served as a tech demo for the console. Some stars also felt repetitive, like "fight the Big Boo... ok now fight the Big Boo again but in a different area." To be fair though Mario games are no stranger to this kind of repetition, especially when it comes to bosses, and you fight Bowser a total of 3 times in the game.
I remember this being a problem with basically all N64 games, but the camera can be a huge problem at times, getting stuck behind level geometry and making it impossible to see Mario and such. I also found the controls to be somewhat slippery. But overall, a solid 10/10 game that is a beloved classic for a reason.