We feel sorry for Nnooo. When it announced Spirit Hunters Inc. in 2010, the 3DS had only just been announced, so DSiWare seemed the best format for the job. As an AR ghost-catching game, it's now clear the 3DS would have been a better fit.
You catch ghosts by looking around with your handheld's camera and tracking them on your radar. Find a spectre and you try to wear them down enough to catch them. It's basically spooky AR Pokemon, right down to Light and Shadow versions, each with its own exclusive monsters.
The DSi's lack of a gyro sensor is a big problem, forcing the game to use wonky object tracking to replicate the sensation of AR movement. But the DSi camera isn't up to the task, and the illusion of creatures existing around us never takes root. That said, you can't blame DSi for a horribly fiddly interface that maps attacks to pixel-wide icons.
Perhaps when it was conceived in the pre-3DS days Spirit Hunters Inc. was a fun diversion, but since then we've had a proper gyro handheld bundled with a game that works better than this - Face Raiders - as well as Spirit Camera for those who fancy spookier AR thrills. We really wanted to like this but, sadly, it arrives too late and acts as a ghostly reminder of a time when AR games were clunky and didn't work well enough.