To be honest, I really do not have that much an opinion on Mario Circuit since I do not play on the stage that often but when I do, Mario Circuit can become a very fun stage to play on for Brawl.
The best parts with Mario Circuit is, the music and animation is well done in the stage. Every single song that plays on the stage is recreated well and enjoyable to listen to as the music in the stage ranges from the music of Waluigi Pinball Track from the DS to the classic SNES Mario Kart music in the stage. Animation and Music wise, Mario Circuit is one of the best tracks in the game.
Gameplay wise while not as great as the music and animation, it is still good in Mario Circuit. Like Lylat Cruise, you do not have to worry about cheap gameplay elements getting in the way on the stage. The only gameplay element that happens in the stage, is the moving Shy Guy Karts that appear sometimes in the stage but, thankfully the karts do poor damage to you and your opponents and they are easily predictable to notice and easy to dodge on while fighting on the stage.
If there is a problem with gameplay is that like Lylat Cruise, the stage is fairly small to battle. More stage battle room would be nice in Mario Circuit but, overall, Mario Circuit is a fun and good stage to play on in Brawl. Mario Stages so far in the series, are enjoyable to play and fight on in Brawl.
