I never ran into this game in a U.S. arcade, though its 1984 vintage means it would have come out during the "crash era" when arcades were closing and home systems went into hibernation. According to Wikipedia, it was released in US arcades through Romstar, and converted to the Nintendo Famicom, but never came to the NES, though the coin-op version arrived on the Wii Virtual Console in 2010. At any rate, this was my first opportunity to play SonSon.
The storyline is conventional -- our heroes Sonson the monkey and Tonton the pig (lead characters in the Chinese story) are out to rescue their friends, who have been swept up and kidnapped by a cloud-riding warrior villain character; there are definite echoes of this opening sequence in Capcom's slightly later Ghosts'n'Goblins.
Gameplay consists of running along a forced-scrolling set of six layers, hopping up or down and firing left or right to take out enemy creatures while collecting vegetables and other treasures, as we try to fight past 20 milestones to reach the end of the adventure. There are gaps in the layers which aren't fatal but force the player to drop down a level, and there's no way to stop the scrolling. Most enemies approach on the horizontal levels, some walking and some flying, while other enemies jump out of the waters below. The jumping and flying enemies can usually be avoided altogether by limiting movement to the upper tiers, but the ground-based enemies have to be dealt with more directly.
The game features a two-player co-op mode; I couldn't detect any differences between the two characters in terms of control or abilities. Both can fire shots to the left or right, and hop up or down one level of the scrolling screen. When we're out of lives it's Game Over; the marketing trick of allowing players to continue with additional credits hadn't yet been introduced.
I got this far before dying in my best go -- nowhere near the end of the game, only about 25% in, but far enough to feel like I'd experienced a good portion of the action:
What strikes me most about Son Son is the way it mixes up the action without interruption -- ROM chips were becoming less expensive, and the additional storage allows for visual variety from one zone to another. The gameplay remains largely the same, with different visual styles and hazards in each zone, but the relentless pace keeps the pressure on, as there are no pauses or breaks in the action -- we just scroll from one numbered zone into the next, and we have to keep running, shooting and avoiding enemies (there are brief stoppages when we encounter a fortress, but even these events are timed -- if we don't finish destroying all the enemies, the screen starts scrolling and we're back in action.)
The most annoying thing about Son Son is that there's not much of a "time out" when the player character dies -- we get thrown immediately back into the action, riding in from the left side of the screen on a cloud, and if we happen to be in the wrong place at the wrong time when we take control again, it's not hard to die instantly as the screen has become overrun with enemies in the meantime. We have to do well with our first life, because (in my experience) we're likely to lose one or two after we lose the first one.
Bottom line, SonSon seems like a typical early-to-mid-1980s arcade game -- cute, colorful, simple graphics on a black background, with a fair amount of challenge and a graded difficulty curve. The basic mechanics aren't hard to master, but memorization becomes important as the levels progress and enemies enter with little warning or time to react. The visual changes keep the game interesting, but the gameplay becomes repetitive after a while, and the side-scrolling genre would evolve far beyond SonSon within a few years after its debut. Entertaining, but not addictive or unique enough to feel like a "lost classic." I enjoyed spending a few hours with it, but it's not a cabinet I'd be eager to have in the house.