I have purchased extensively from all three of the major app stores, and I can say with a fair degree of certainty that the Android app market experience is the worst of the bunch. They have comparatively lax oversight, so all kinds of garbage gets in. In addition, navigating their market is not fun. It was actually not bad before they changed the look of the app store to a more graphical interface, but it's now much laggier because of it.
The Apple app store is probably the best. Developers focus on their iOS version more than on that of other OSes, and it shows. Also, the store itself is intuitive and searches typically yield smart results. Our Marketplace falls somewhere in between the two. We have the fewest overall number of apps, but per capita, we arguably have a greater concentration of quality apps than Android, although Apple may still be ahead of us there. As far as the interface of our app store, it's better than Android's, but Apple seems to have more polish and a smarter search algorithm. Granted, these are just my personal views, but I think they're probably representative of the whole that has appreciable experience with all three.