One of my favorite racing games of all time is F-Zero GX for GameCube, and although I've dipped into its predecessor, F-Zero X, for N64 several times, it was hard to regress from the perfection that was F-Zero GX.
I finally sat down and played through the three main cups (Jack, Queen, and King) and tried out the three main different difficulty levels (Novice, Standard, and Expert). I was relieved to see that the game is much more like F-Zero GX than the original SNES F-Zero game. F-Zero X introduces a whole batch of new drivers, vehicles, locales, music, and modes, and although the graphics are indeed pretty bare, the game runs completely smoothly and does provide a great sense of speed. So it's an improvement on the SNES game in every way, except for the fact that each cup is six tracks instead of five (although each race is three laps rather than five as in the original).
The game also introduces the ability to boost continuously from lap two on at the expense of your shields, and the ability to attack rivals. In this game attacks are more awkward to execute than the GameCube game (requiring multiple button presses instead of one), but they're still fun to do. The AI is much better in this game than in the original, and rival racers don't relentlessly attack you. There's a satisfying trade-off between boosting and having enough shield to survive bumping into the track barriers and other drivers, and there's a lot of nice variety in the tracks. The tracks with sections without barriers can get to be fairly frustrating on higher difficulty levels, though.
Basically the game feels like exactly what it is: a fun and enjoyable precursor to F-Zero GX that is inevitably overshadowed by that game's greatness. I've never been that into racing games, and as I've gotten older I've gotten even less inclined to play them as much as you need to in order to fully memorize all the tracks, but I enjoyed my time with F-Zero X and wouldn't mind picking it up again at some point.