If that's because Android's interface is laggy, then why do people keep buying Androids?
IMHO, iOS users spend more on apps because iTunes is better than Android's Marketplace - this may have been a deliberate decision by Google nonetheless. Did you know that I am not allowed to sell Android apps in the Marketplace because my country is still not approved for registering a Google Merchant account? That's right, it's almost 2012 and while I can sell apps on iTunes, I can't on Android's Marketplace. However I have no problem integrating AdMob in those apps. So it's like Google wants ad-supported instead of paid apps, which considering their main business model it isn't that shocking.
However, you can't say that Android and its UI is not a success.
People keep buying Android for lots of reasons, they can be a lot cheaper, they have more options (keyboards, big screens), they are pushed a lot by carriers, lots of carriers don't carry the iPhone.
The interface problem with Android might come in to play when people are considering their second smartphone, and devices are a lot faster these days. Of course, no one should need a dual core or quad core CPU for a smooth experience on their phone.
That doesn't answer the question, which has a context: if people don't buy Android apps because the interface is laggy, then why do they keep buying Androids?
I have an iPhone 3GS and a Galaxy S which recently upgraded to Gingerbread. The iPhone experience is indeed more "smooth", however that doesn't bother me because the advantages that my Galaxy S gives me outweigh the smoothness of the iPhone.
As an example, on my Galaxy S I was free to install a calls / SMS blocker. You can blacklist certain numbers and then it's as if those numbers don't exist - not only it blocks the calls / messages, but it also removes all traces from the logs. Last time I checked these types of apps where banned from iTunes. On Android it isn't so cut and dry either, as these apps are using private APIs that aren't documented, however they are allowed on the Marketplace.
Another example would be the kickass integration my device has with my Google Apps account. For instance all my phone numbers are synchronized with Google Contacts. And don't get me wrong, I'm sure this is only a matter of preferences and I could probably do the same thing on the iPhone, however this works both ways and I'm having an easier time with my Android to do what I want.
The only annoyance I have with my Galaxy S is the slow upgrade cycle. It will probably take forever for Samsung to deliver to me an upgrade to Android 4. Which is why my next phone will be a Google Galaxy Nexus, or whatever blessed phone will come next after it.
IMHO, iOS users spend more on apps because iTunes is better than Android's Marketplace - this may have been a deliberate decision by Google nonetheless. Did you know that I am not allowed to sell Android apps in the Marketplace because my country is still not approved for registering a Google Merchant account? That's right, it's almost 2012 and while I can sell apps on iTunes, I can't on Android's Marketplace. However I have no problem integrating AdMob in those apps. So it's like Google wants ad-supported instead of paid apps, which considering their main business model it isn't that shocking.
However, you can't say that Android and its UI is not a success.