Not brilliant, but well worth it.
The first thing you notice when you open up the game is Story Mode. This is the most disappointing part of the game. The story is based off of the F-Zero anime (yes, there is one), so the characters either have altered back-stories (Zoda works for Black Shadow? Blood Falcon killed Jody's brother?) or are brand new characters (i.e. Rick Wheeler or Lisa Brilliant). The stories, also, do a mediocre job of relaying the characters, and a hackneyed job of telling the story, which is spaced out over 8 characters. Also, most missions are very easy, with the exceptions of Samurai Goroh and Black Shadow, who are very hard due to poor turning. It's worth going through to unlock other racers, but it's not very good.
But even in the Story Mode, the cars handle very well and hint towards where the real value of the game is: Grand Prix. The game play is simply incredible. Turning is very responsive and feels natural. The sense of speed is excellent, especially for the GBA. Also, the amount of A-tapping to steer has been cut down dramatically from Maximum Velocity. Further, the difficulty is perfect, from very easy on Beginner to very hard on Expert.
Another plus is the amount of racers available. F-Zero Maximum Velocity had only 10 cars total. GP Legend has a total of 34 racers, each with their own unique feel, adding incredible depth to the game. It also includes your favorites from F-Zero X and GX, such as Red Gazelle, Twin Norita, and, my favorite, Little Wyvern.
Furthermore, the sound is, simply put, epic. It includes several of the best F-Zero themes, and all of the music fits well with the races. Graphically, the game is great for a GBA game, and much improved over Maximum Velocity.
In the end, the verdict on F-Zero GP Legend is simply "Your mileage may vary." If you like F-Zero, then this is a great game you should pick up right away. If you don't like F-Zero, then pick it up if you find it cheap. However, from this F-Zero fan, this is one game I'm very glad I got.