I do quite a lot of flying with just the iPhone - just slip it in the pocket and Go!
However, the iPad fits in the flight bag with almost no extra space and is so much nicer to use. Also, I buy my holiday reading as ebooks (along with dragging my required work reading along as PDF/Word files
) so one thin iPad is a lot better than a ton of paper.
I have a desktop link to the Nats site for narrow route briefings, a Google calendar for the plane, a couple of weather apps and use Rocketroute or EuroFPL for filing (Rocketroute until very recently was IFR only but now supports VFR filing as well).
As an aside, I have also load a PDF version of my maintenance manual and parts listing onto the Ipad to help locate spares on the road.
At the moment, this means I need to do my flight planning before hand on a computer. I do the route generation, initial Notam brief, a Plog using forecast winds and a trip chart - print these as PDFs (which I load into the iPad) and paper copies. This works as long as the return is within a couple of days. If longer, I do a zero wind plog. On the day, I just check weather and go to the Nats site for an update briefing (which just gives you new notams since your previous request).
I do still have an old tablet computer that holds an out of date copy of JeppView (never use it anymore) and a current copy of FlightPlanPro.
And I even have a stack of reasonably recent AIDU diversion plates - which I once had to use on an IR check ride when someone's gear collapsed at the field we were flying from and I needed to 'divert' to give my examiner a ride to his next assignment!