It's an underappreciated masterpiece (for a sonic game). I'd rather Sega kept putting out games like Shadow the Hedgehog than braindead games like Sonic Colors. But oh well. :(
It's a masterpiece compared to most other Sonic games, especially the more modern ones. Other than Sonic Adventure 2, I can't think of a 3D sonic game better than Shadow the Hedgehog. With how low the bar is, I think it's fair to call Shadow the Hedgehog a masterpiece in the realm of Sonic games.