Here's the short version: Ginga Tetsudou 999 is fun and gorgeous and absolutely worth watching.
The most obvious appeal is the visuals: it's directed by Rintaro, and you can tell. Every single sequence in the movie is beautifully animated and expressive. The big action set pieces are just plain fun to watch if only to marvel at how great they all look. The opening scene is in particular a standout, especially because it (like the entire film) has beautiful character animation that gives you an immediate sense of the personality and attitude of every key player. Tetsuro in particular is instantly likable, even before you learn anything about his backstory, purely because the animation makes watching him move and talk and make faces so enjoyable. Once the film gets going, Rintaro uses character animation judiciously to develop Tetsuro and Maetel's relationship, which is just charming and lovely to watch. The beautiful settings and environments, especially
A contrast with the Ashita no Joe film is illustrative here: both are movie adaptations of very long classic 70s anime, with excellent (though very different) character designs by Sugino Akio (who also designed a number of other classic 70s shows and would go on to do work on Black Jack and, most famously, Legend of the Galactic Heroes.) 999 makes the smart decision to cut the majority of planets and leave behind the most powerful ones, and use character animation to fill in the gaps left behind in the key dynamic of the film. Ashita no Joe's character animation is great too, and the film similarly uses it to characterize its key players, but unlike 999 it doesn't rely on this animation to do the heavy lifting and instead adapts many of the key plot beats and details from the TV series. The resulting movie ends up feeling a bit plodding, which 999 largely avoids.
That's not to say the film is devoid of flaws: you can definitely still feel at times the limitations of adaptation, and many side characters are introduced and developed but not really tied into the conclusion, and left feeling interesting but ultimately a tad superfluous. The movie attempts to graft the core plot of the show together with episodic adventures in a way that doesn't quite manage to feel seamless, and I'd be lying if I said I was particularly interested in the core themes or came out of it feeling impacted in a way more substantial than "That was fun!" Still, fun isn't at all a bad thing to be, and Ginga Tetsudo 999 does it very well.
