I have been looking for a cheap cover for my Korg Trinity for some time. When I got this bag, it fitted perfectly and protects my keyboard against dust from all sides.
The cover certainly would not protect against water so I would recommend its use if you are playing outside a studio, neither does it have any form of padding so it wont protect against anything heavy being dropped on top of your gear. Instead, it is a simple dust cover made of some synthetic, non stretchy, fiber that does the job of covering (not protecting) a keyboard or synth about the same size as a Trinity.