Just because I had to. Upon entering the theatre, I knew the movie would be either good, or bad, but didn't care, as I felt it my duty to see it once, give how much Speed Racer entertained me in my childhood. ("You shall not race in this race, for this race is dangerous! And dangerous races you shall not race in! Grr!")
Overall, I thought the dialogue was extremely cheezy, but the racing scenes were absolutely amazing. Though the official races might cause seizures with the intense lightshow going on.
Quite frankly, I think anyone should just see it for the hell of it. It's one of those movies that, good or bad, you're glad you saw it.