I was initially sceptical about how useful I'd find an iPad but trying out a friend's iPad convinced me it would be transformational. When I started using one, the crossover occurred with both my MacBook Pro and my iPhone. I was doing similar things but in a more convenient and enjoyable way. Plus the iPad lent itself to new activities like reading books and watching TV shows which previously I hadn't bothered to do on mobile devices.
I recently won an iPod Touch, a device I wouldn't have considered buying because of the overlap with my iPhone 5. However it has proved surprisingly useful as a media device. I don't need many apps on it, leaving plenty of space for more music and video than I can fit on my iPhone.
The iPad Mini intrigues me because although it replicates the iPad in terms of functionality, the smaller screen size introduces another dimension. As an ereader the full size iPad is big and heavy. The iPad Mini is much more user friendly in this space. Travelling on the train also feels like an occasion when a smaller iPad would be more convenient. Will the lack of a Retina display be an issue here? Not sure, although the rationale for no Retina display in terms of performance improvements seems a reasonable trade-off. It remains to be seen whether the iPad Mini will outsell the iPad over time; when it gets a Retina display I think it will. I suspect the iPad Mini will prove transformational in use - and if I get one I'll let you know!