I just finished watching it tonight and I've got to say I thought the movie was really good. I grew up with the show and perhaps that gave me a perspective that younger audiences wouldn't have, but I entirely "got" the treatment and thought it worked very well. Speed Racer as a grounded concept is so ridiculous I couldn't imagine the idea plastered in reality without coming off as straight camp. This treatment gave you a surreal quality completely fitting for the concept and certainly the Mach 5 was every bit as versatile as the cartoon version. It would have been insane to imagine the Mach 5 as a REAL car performing those stunts. But then again television audiences didn't seem to have any problem believing the General Lee could survive all of those jumps, even though you could see the front end collapsing just before they would edit to a new scene with the General driving off unscathed, so who knows?
Anyway, I thought the characters were cast very well, especially Speed and Racer X. I loved John Goodman as "Pops" although I did keep waiting for him to turn to Speed and say "Shut the (bleep) up..." (as he did in the Big Lebowski). Spridle and the monkey were hilarious and I thought the story made complete sense and was told very well throughout. I guess there's a certain degree of disbelief you have to suspend when you sit down to a film like this. But I was very surprised at how well I liked it after hearing all of the bad press on the film. I don't think it deserved it in the slightest.