Samsung Galaxy SII: PDA/house brain. It controls my apartment; turns off the lights when I leave, my computer when I go to sleep, notifies me of my grocery list when I go out, other todo lists, gives me access to temperature data from sensors around the apartment, reads me weather and other info when I get up, etc- most of it fully automatic. I don't often leave the lock screen, as I've set it up to either do it automatically, or so that I can do 95% of what I need from the lock screen.
iPad mini: Actual app interaction. For studying, and for teaching. Take notes, read, handle documents, grade papers, digital whiteboard, presenting tool, read magazines, news readers, etc. Pretty much an "app island paradise"; lots of single apps that serve a specific purpose, without the complete tie in to everything and anything that my phone has. I do have a folder full of icons that send commands to my phone, which in turn can e.g. forward its last notification to the iPad, turn off the lights, or check the weather sensors and return the data as a notification on my iPad.
Desktop PC: Alpha and omega when at home, doing work, watching video, playing games. Always on when not at home, allowing me to Splashtop into it from my iPad.