That directly contradicts my own experience with Android. I bought a HTC Hero as my first smartphone, so I didn't have any reference points to compare it with. Over 18 painful months of owning that phone I never got used to the lagginess of the UI. Not to the point where it faded into the background, anyway: I learnt to anticipate the lag, but it never stopped being annoying. So much so that when it came time to replace the phone, there was no way I was considering an Android phone again.
Basically, I'm an existence proof against your argument (and now a happy iPhone owner too).
There was a dark period in Android's existence where a very immature OS came together with underpowered hardware (the GPU in an iPhone 3G demolished the GPU in the Hero) and created a pretty bad experience.
That wasn't what I was talking about. I'm talking about almost imperceptible, frame-or-two jitters that is the current state of Android on virtually any contemporary device. The top selling Android devices are all "buttery smooth" in the perception of their users, even if relative to an iPhone it is herky jerky.
I have iOS 4.3.x on my iPhone 3G and it stutters compared to iOS 3 at the same time. So much so that when I got my iPhone 4S I noticed how much the iPhone 3G lagged and stuttered that I no longer use it, even as a simple iPod.
Fair enough, I haven't tried Android on a more recent device so my experience is certainly out of date. But when there are this many people saying there's a problem, surely it would make sense - especially for a fundamentally data-driven company like Google - to do some user studies and quantify it? I haven't heard of any such studies though; do you (or anyone else) know of any?
Basically, I'm an existence proof against your argument (and now a happy iPhone owner too).