The Q is what exactly you are trying to achieve.
Pre-flight, the job is well defined: route planning, notams, weather, airport contacting for PPR/PNR/etc.
This is easily done with a very common item known as a "laptop"
You can get them from £100 to £2000, and a normal windoze one will run everything needed including - at the top of the GA food chain - Jeppesen stuff.
Airborne, people look for different things. A VFR pilot might want to run a decent GPS moving map (something better than you can buy, or something which can run downloaded/"shared" maps ). An IFR pilot might wanto to see approach plates. Both might want to get weather data, if it is a long flight.
The platform for this is less clear, largely due to a near total lack of sunlight readable hardware which is light, thin, etc. A lot of people are looking at solutions but most of them deliver only a part of the job, so one would end up with a pile of gadgets, which need charging, etc.