I use BB Z10. I have had all alternative smart phones over the years. And tried the mainstream iPhone, Android and WP. And I have to say they are all at some point less than fluid, have some lag. Even the iPhone has some lag sometimes. The BB10 NEVER lags, is ALWAYS very fluid. That's what I like about BB10.
And also that the build in PIM apps are excellent. The best PIM apps I have seen since Palm OS. The multitasking also works very well. I don't use android apps much on my device, but is even possible to install apps from Google Play Store if you want to, aside from the Amazon Store.