I watched the movie twice (once in IMAX). I read the reviews before I went, but I had my mind set on seeing it after I saw the trailer's racing scenes.
I was expecting an almost F-Zero-like bright colours flying by rush and that was just what I got. It was awesome and I wasn't able to sleep for at least a couple hours after. Every time someone asked me about the movie I was just like 'dude... like wow.. it was awesome..'. *Really* hard to say if I liked this more than Iron Man though.
I loved all the CG to death, for me it was the more colours the better. I didn't like the chimp and the kid scenes much though, but then, they were really short.
I have not ever seen an episode of the original Speed Racer, but my friend who went with me had said it was quite similar to the show. I think the reviewers are a tiny bit crazy because this movie does NOT deserve the low rating it got.