There is one interesting notion about taking a photo on a tablet....the image you see on the screen can be large enough to tell whether it is in focus or not. I have gotten into the habit, when using my 4S for photos, which I do often, to take 3 or 4 photos just to ensure I get a couple that are in focus. I'm thinking the mini might be decent enough, with its 5 mp iSight camera, to be useful (well, admittedly I still can't (or won't) whip it out of my pocket at 5:30 am in the morn, in the dark, to snap a photo showing that I got 50 mpg on my commute to work. But I might think about using it in other situations...we'll just have to see how that goes.
I'm also wondering if the size will be such that it would make sense to use it as a secondary Nav and to play audiobooks and music on, instead of my phone. Yeah, the phone is with my all the time, but man it is small. Trying to do anything with it other than make/receive calls is just dangerous, in a car. I wonder if I can link two devices into my car's BT? Hmm.....
Is there any possibility that there's something wrong with your iPhone camera? I've used iPhone 4 and 4S to take thousands of photos around the world (and now use Samsung Galaxy Note) and haven't had probs with focus unless I'm driving and try to shoot, which I'd expect.
Or is focus something that's been an issue in general for you? Is it a matter of not being able to see the screen or hold the phone stable? One possibility that might help: Before you snap, hold your finger on the button, but don't release till you're steady and ready to shoot. That can help focus (and works on various cell phone cameras and other types) before shooting. I suggest trying that, because if you can focus on a cell or digital camera, your photos are going to be so much better than with iPad cameras just because the cameras are so much better in quality.
On iPhone 4 and 4S, the only time I needed to take more than one shot of something is if I accidentally moved my hand and messed up focus, wanted to correct because of people's expressions or eyes closed, or if I was in a car and got a blurry result. The results were so good that I haven't carried a separate camera in years.
Of course, if focus is a prob, you've got to use what works best for you.
About car use, just personal pref for me: I still like standalone GPSs, and I use a separate device for audiobooks and music, whether Samsung Galaxy Note phablet, iPhone or touch. I use iPads or Kindle Fire original 7" usually only if reading or surfing the web, or to look up travel info or maps, but when I'm a passenger. I find iPad or Fire 7" too big for car use otherwise. For instance, if I'm changing music while driving, I like using a smaller device that I can hold and scroll in one hand. But that's just me.
For phone calls, my GPSs let me BT my phones. But I rarely use that. I don't usually like to chat while driving.
I could see either 10" or 7" being a pref for some, especially if eyesight or touch is a challenge when driving, or maybe someone just wants a bigger screen.