Steamboy is indeed such a quintessentially and self-awarely steampunk movie that it should be the go-to work for anyone unacquainted with the genre and curious about it. It stands in sharp contrast with the dismal failure that was Wild Wild West, which was all chaff and no wheat.
I too found the second half a little exhausting, one climactic action scene on top of another. Otomo was clearly enjoying himself with his toys but one gets the feeling that he went ever so slightly overboard. The sort-of epilogue showing where the characters go from there, personally and technologically, was a nice touch though. If would be fun to have a sequel involve a World of the War-esque Martian invasion.
I remember gobbling this film up as a teen in love with all things Steampunk. I think it’s to Steampunk film what something like Boneshaker is to Steampunk novels, a significant codifier and trope compositor that runs on a lot of rule of cool (though I think the Boneshaker series is better because the writing is better etc.)