After years of using different Nokia (phones), and Blackberry, I started using an iPhone 6 months ago, and an iPad since it was released. I still keep all 4, with 3G cards in each, because each has its area of optimal usage.
The Blackberry is the best for quickly responding to or forwarding emails where more than a dozen words are required, or while driving or in public transport. Nothing beats it for speed and ease of use. But as an internet terminal or otherwise personal device, it is terrible.
The Nokia is the best phone, and 3G modem for a laptop. No smartphone comes close to the sound quality and network connectivity of the Nokia.
The iPhone is a pocket portable internet terminal and can run a huge range of useful applications. The iPad is a backpack portable iPhone with a large screen and without regular phone functionality (though Skype works well) nor a camera.
Does the iPhone make the iPad irrelevant? No, very much complementary. I work (and read and play) constantly on the iPad, and I keep the same content in my pocket for quick access anytime. By selecting apps which are identical on both platforms, I have the advantage of a comfortable tablet and the pocket portability of a smartphone. Everything I was doing with the iPhone is now much easier with the iPad.
If I had to choose fewer devices, it would be the iPad, with a Nokia phone, and optionally a blackberry and/or an iPhone.
The laptop and home computer have been resting much more since the iPad arrived, and that is a trend that will continue as tablets become more generally available.
This post is way off topic for my blog, but perhaps it will be useful to some others with such questions.