A fair bit better than I've probably been giving it credit for all these years. It's still aggressively bland from an aesthetic standpoint - just nothing interesting going on at all - but mechanically and as far as level and overall game design go, it's quite strong.

Upon starting it (especially if coming right off of previous Mario games, as I just have) the controls feel very slow and slippery, which is, of course, a necessity given the smaller view of the handheld and how much of the screen Mario now takes up. But if you stick with if for just a little while, you can quickly get dialed in and discover some of Mario's exciting new abilities, like the really good, chunky wall-jump. Another issue you have to retrain your brain on a bit is the hitboxes, which are always going to be worse with 3D models, especially low-count ones like these. But if anything, they're generous, with you being able to fully clip into Thwomps, etc. a little bit without them killing you. You just gotta go with it and trust the behind the scenes math - Nintendo's guys still know what they're doing better than anyone when it comes to platforming, and in the end, it all works just fine.

On top of some overall top-notch level design (which features a shift further into breaking each stage down into sequences of discrete challenges, reinforced by the offshoot feats needed to get each level's three big coins - a great addition, imo), I also really like the structure of the world maps. Along with the big coins, the more prevalent branching paths design folds a kind of minor collectathon aspect into Mario, evolving the exits from SUPER MARIO WORLD and expanding it into a meatier completion system. It's great! You can mainpath it to the end pretty quick even without warping if you want to, but you're not going to see nearly all of what the game has to offer if you do. I think this is one of more rewarding Mario games to 100%. (I mean, not in terms of unlocking anything meaningful or whatever. Goodness, no.)

Aside from the beyond-boring look and sound, and little lamenesses like the nothing final boss and a couple too many autoscrollers (although they do each have unique spins on the idea, at least), the game is quite good and lives up the legacy of the series well enough. It may have begat a solid decade of dull retreads, but I guess I probably shouldn't let that skew how I feel about this one specifically.

Best of the "New" series definitely. It's the first, and only one that tried doing so many new things regardless if they stuck or not.

Here's a list of some things that were never in 2D Mario before this game: Star coins, triple jumps, wall kicks, ground pounds, player 1 playable Luigi, bonus minigames, mini and mega mushrooms, fake final level, and many other one off gimmicks or minor things that are now mainstays Mario.

The bosses are also the most visually unique, even if not very challenging, Monty mole tank went hard.
I also really respect that they had the balls to hide 2 of the games unique 8 worlds behind a secret method, when I was a kid and found them I felt I had done something no one ever had, it was so cool.