Because of ergonomics, tablets will be more successful for content consumption type of activities like eBooks or entertainment, primarily on bed or sofa, but not at desk.

All input on touchscreen including innovative gestures still requires users to place their fingers directly onto their eyeballs: either with head looking down or with hands rising up. Both postures are not very productive for programming or desk jobs and can't sustain for long hours.

iPhone is more successful because of light weight and more sensors. To make tablets usage experience better, Internet tethering and sensor sharing via iPhone will help a lot.