I use a 2nd hand Samsung Tab 4 with an 8" screen. It is perfectly ok, and only cost £80. It also doesn't need an external GPS receiver, and doesn't have retina display.
Before I used SkyDemon I would plan a flight the night before to save time. On the day I would then only need to add in the wind and calculate. I would then re-read the notams and weather just in case. With SkyDemon I do the whole lot in about 15 minutes on my computer, and then save it to the cloud.
cjm, I completely understand how you feel you're cheating, and so do I. After all planning a flight shouldn't be that quick and easy should it?
I also make a point of retrieving the saved plan to my tablet before I leave home, so that I can also download the aerodrome plates, including any alternates just in case. Of course I could do all of that at the aerodrome using their WiFi, but I just prefer to do it at home just in case the aerodrome WiFi isn't working.