When it's used appropriately, it's effective--a lot of fighters seem to throw it when their opponent moves back, which is pretty much useless. If you catch them coming in, or with their weight planted, it works quite well for stopping their forward movement. Depending on your target, you can injure the knee (as we saw with Miguel Torres), or you can cause pain and start wearing away at their leg just like you would with other leg kicks. It is also very useful at close range, but I have yet to see MMA fighters use it in that context. A man I know who teaches police, military, and self defense courses has had three students break people's legs at close range with oblique kicks.
Now, I will admit that there was a time when I didn't think oblique kicks were worth anything. They just didn't seem like they had the range or power to do much good. Then I sparred with a guy who was good at them, and used them extensively, and I found it very difficult to defend against. By the end of the round, I could barely walk, and I had been wearing pads! Ever since then, I started really exploring them and utilizing them in my training.
