It's easy to get excited about Onslaught for the simple fact it's a first-person shooter and there aren't many of those on the Wii.
Thankfully, it's also a good little game... as long as you're playing it online. The single-player Story Mode is extremely dull with a plodding pace and samey enemies (not to mention an incredibly annoying line of dialogue that repeats almost every time you kill an enemy).
There are a few nice touches - if you kill an enemy close-up and get their goopy green blood on your visor, it stays on the screen until you shake the Nunchuk to wipe it off with your hand - but it's still a very average shooter.
However, when you connect to an online match (where you have to score the most points by shooting more enemies than your teammates) it's really enjoyable. As a first-person shooter Onslaught is never going to trouble Metroid Prime 3 but for £7.50 it'll keep online gamers happy.