The iPad is better than anything else around, and it's not got much to do with being proprietary. The only serious contenders are F/OSS-based: Android and WebOS.
By saying it does, Miguel is engaging in the same kind of cargo cult that has led Microsoft to try to compete with Apple by copying the wrong things. Take Zune and its closed platform ecosystem, the restrictions on Windows Phone 7 development and so on.
No, Apple has succeeded better than anyone because Steve Jobs is really good at industrial design. By industrial design I do not mean making nice shiny objects. Slapping a fancy 3D interface on a piece of shit is not design; hiring an expensive designer to draw the packaging and enclosure of a shitty product is not industrial design.
It's putting all those things together so that the product is robust, does not frustrate the user, affordable as opposed to cheap, and, most importantly, does something useful for the user.
(Apple products are often seen as way too expensive compared to the competition, but the fact is, for the past 20 years, they've had excellent resell value even a few years after release. How much can you sell a 3 year old Dell laptop? It's probably not even worth the shipping; while you can resell a Macbook that old for a reasonable amount of money. It's not cheap, but it's valuable.)
That's why the battle of specs competitors are trying to wage is incredibly retarded. Take the front facing camera that the iPhone 3 and 3GS were lacking; almost all high end smart phones at the time had them. I had one on my N900. *None* of them had decent software to support it. What good did it make? Entirely pointless.
And again, this has nothing to do with proprietary software. The only way proprietary software serves Apple here is because they have a competitive advantage. But the market is becoming a commodity. The iPhone has less of an advantage (it's still much better than the competition but the gap is closing), so it's going to become pointless, in fact, a hinderance: openness will be an advantage to Android when the playing field levels. The corollary is that Microsoft's fucked, they're not better than Apple now, they could catch up but they will never be significantly better than Android.
On new markets, like that of the iPad that Apple created, it serves Apple right now. In time it will matter less, in the same manner. And again, Microsoft is in trouble.