Contrary to what other reviewers may say, the sequel to the original cartoon/manga works out well.
The problem with sequels is that so many people hold the originals to such a high regard because the originals were usually executed at an early time in that viewer's life. It's hard for such viewers to admit that they weren't always as critical or as intelligent in their youth as they are today.
Going back, I re-watched some of the original Speed Racer cartoons, and having little previous memory of them I was able to watch them with a critical eye.
The original cartoon really was weak from a script, animation and direction standpoint. This isn't surprising since it was in an era where stock animated cells were used often in the industry (exactly how long are the hallways in Hanna-Barbara cartoons anyway?).
To say that the execution of the Next Generation is weaker than the original series really is like saying the special effects of original star trek were better than say in Next Generation, Deep Space Nine or Voyager.
I use the Star Trek comparison because I don't want people to think I'm bashing the original Speed. The original speed is unpolished, some scripts were fun, others weren't. Original Star Trek is unpolished, some scripts were fun, others were downright terrible. So, Maybe you don't like TNG for Speed or TNG for Star Trek, that's fine; but don't blame the script quality or the special effects for your dislike. Fact is Original Speed used quite a few stock scripts, generic stories of the time with no real originality. Same with original Trek (just take them out of space and put them on "Wagon Train", you'll see what I mean). SR:TNG mostly tried something new, same thing with ST:TNG.
One big plus to the series is unlike the movie, Speed Racer: TNG does NOT re-write speed racer history. It adds to it. Nothing changes a single episode of the original series.