After much deliberation, I bought an iPad mini last weekend. (16Gb, cellular model.)
My rationalisation was that my iPad 1 is no longer getting software updates, and although my iPhone 4 is over 2 years old (and therefore out of contract and due an upgrade), the only things I want to upgrade it for are Internet/app related (as opposed to phone-related.) and an iPad mini is a better and cheaper 'mobile Internet' device than a new iPhone.
In addition, data costs work out much lower- in a typical month, I'm spending £16 on 1.5Gb of mobile data on a 3G phone plan, while for the same cost on an iPad plan, I get 5Gb and 4G/LTE data speeds. (I think the reason is that phone data is so tied into phone usage - in a typical month, I might make 40 minutes worth of mobile calls, but I'm paying for a lot more. Partly in case I need the minutes, and partly because in the past it has balanced out with cheaper handsets/data.)
What I'm interested in logging is how my expectations meet up with my actual usage. I'm expecting (hoping?) that this will become my 'main' mobile internet device — so my iPhone will become something I just use for phone calls, text messages and maybe listening to music and podcasts. (The three things I can't do with an iPad mini are make phone calls, send and receive SMS messages, and put it into my trouser pocket. Although, with Skype and iMessages, I could do some amount of SMS/phone functionality with the Mini, the 'fits in my pocket' feature is really the killer app for the phone.)
I'm also expecting that, with both the bigger screen and bigger data package than my iPhone has, that I will be using it much more for video than before (where before, I was either waiting until I got home before downloading video or loading video onto my phone which I never got around to watching.)
Now, I know that I'm not going to want to replace my iPhone with something that doesn't give me access to the web, emails, maps etc. While an iPad mini fits into the pocket of a winter coat, or comfortably slips into my bag, there are plenty of times and places where the phone's form factor is a better fit for what I want to do. But I'm expecting the iPad mini to take over most of my mobile data usage.
What will be interesting to see is whether I still have any need for an iPhone in 6 months or so - if all I want from a phone is basic web/email/maps access, then I should be able to make do with a cheaper Android phone. So it will be interesting to see if an iPad helps me to 'cut the cord' with iOS for phones — or whether an iPhone 5S/iPhone 6 can convince me that an Apple-free phone is something I can't live without…