I've been rocking my DS more than any other system lately and I've really been trying to dig up lesser-known gems... Back To Stone is a GBA cart that seems to meet the criteria so far. The gameplay is quite satisfying, and I haven't heard a peep about it anywhere, either in magazines or online. I haven't even seen a single copy in stores. In fact, the only way I knew about it was that I saw it posted in the New Release section of GameFly's web site—as far as I can tell, they seem to have every single game released domestically.
A third-person isometric action game, BTS sets up a post-apocalyptic game world where demons have taken over, and a small group of humans are resisting enslavement. The story is pretty lightweight but it does frame the adventure effectively, and for a handheld game like this, it's pretty much all you can ask for.
The bulk of the game involves bashing demons with your fists, turning them into large rocks (hence the title). These rocks only exist for a short time, and they're used to solve small, bite-size puzzles that litter the environments. It's nothing revolutionary, but every element is nailed spot-on and it gives off an extremely pleasing old-school vibe. In fact, it reminds me of nothing so much as a nouveau-classic Genesis game... if BTS had been released during that era, it would have undoubtedly been a blockbuster.
The graphics and animation are surprisingly good, and the level design is excellent. The developers, Neko Entertainment/Hidden Floor, have done a great job capturing a techno-primitive quasi Diablo-ish feel that works perfectly. Little touches like the main character slowly morphing into a demon as he takes damage adds a lot of atmosphere, and any time a power up is received, it's like a gift from above because the difficulty level can be a little high at times. As a matter of fact, the bosses are on the grueling side, but that just reinforces the old-school vibe.
I'm only about halfway through or so and I think there's a very strong chance that the difficulty might ramp up a little too high for my tastes, but I wanted to give a shout-out to others like me who'd be interested in picking up a GBA gem before it becomes impossible to find... I wouldn't be surprised at all to hear that the production run was microscopic.
... oh, and one word of warning—one "old-school" aspect that I actually don't like is the fact that BTS uses an NES-era password save system. Bring along a paper and pen for sessions on the go, or else you're SOL. Oh, and naturally, when you write one down... WRITE LEGIBLY. ; )