First FPS in Wiiware and it certainly feels like it.
Story of Onslaught is irrelevant. You are a member of a military team sent to search a research colony after contact has been lost. On the planet you face hordes of cyborg insects, which you need to shoot and discover what happened to the colony. There are no cutscenes or voice acting, but text is implemented to tell a story.
Wii controls are typical FPS style and it works nicely. Shooting action is boring, but since there are not that many games of this genre on the Wii it's at least different to shoot things with the wiimote.
Graphics are horrible: there are no details to speak of, environments are like still images and colors are not that many. Sound effects are only a few and music is something you would have expected to hear around 10 years ago.
Onslaught is like blast from the past with functional Wii controls in it. At the very least it has established a beach head for future FPS titles in Wiiware.