If there's any downside, it's that Sin and Punishment doesn't feature true two-player co-op. Instead, the second player is represented by a simple targeting reticule; you're given unlimited ammo, but you lack any special abilities. Third-person rail shooters rarely have two-player co-op, so its inclusion in Sin and Punishment can be seen as a bonus, but as compromises go, it's a bit half-hearted.
Nevertheless, Sin and Punishment: Star Successor is a worthy successor to the N64 classic, and probably the best shoot 'em up on the Wii period. Make sure this one finds its way into your library.