The big difference being that Apple supports their branded products. (More specifically, they're probably not reverse-engineered. I know, it's just a power connector, but there's some facility/protocol for charging the internal battery or not, not to mention that there are four conductors.) If your MacBook power supply dies and Apple discovers that you've had an unauthorized power supply connected, you might be out of luck. Actually, that's probably in violation of the Magnuson-Moss Warranty Act (in the US), but have fun arguing with Apple about that. And I'm not defending Apple doing that. I'm just saying that it's another example of Apple/Jobs telling us that they know what's best. "This is all the power you'll ever need, and I don't even see the people who are telling me that's not true."