Any knife is good for self defense if you know what you're doing and if you don't have something better at hand. The time to buy a knife for self defense is after you know enough about knife fighting to know exactly what you need.
Still, it would be hard for me to think the Onslaught is not a good SD knife, just because if its size and axis lock, which makes it quick to deploy. Just flashing a blade that big will give you a Crocodile Dundee moment so savor. The handle is smooth, with not a jimp in sight, but its designed to fit your hand. And the handle is big, so it's easier to hold securely.
But the real beauty of the onslaught is its blade. It's big. It's wide. And it's thin. It's thinner than even the Military, which is my reference for a good slicer. The Onslaught is roughly the size of the Rukus. Both are high flat saber ground, but the Onslaught is wider and higher ground. The Rukus has 0.8 inches of flat grind at the widest, while the Onslaught is 1.2 inches -- and the grind is coming off much thinner stock.
The thin edge of 154CM lets you get it incredibly sharp. It's the best slicing big EDC I've tried.
I like it for an EDC, but it is big. The blade is more than 4 inches. I carries a little easier than the Rukus, but nowhere near as well as the Military.