You can only judge image quality if the screen you’re viewing is properly calibrated with a hardware device.
Calibration combines a hardware puck with software that flashes known colour values to create a colour profile of your screen. Datacolor and Pantone are the best known brands. With prices starting under $100 it’s a no-brainer for anyone serious about image quality. $200 will get you a professional quality unit.
With more people using iPads for evaluating images it’s important their screens are calibrated. Both the Datacolor Spyder and Pantone X-Rite ColorMunki let you calibrate your iPad.
To do this the computer hosts the hardware puck and wirelessly connects to your device to build a profile for the iPad screen. Very clever.
The iPad and Apps are not colour managed. So you’ll also need apps that are colour managed: Datacolor offers the free SpyderGallery and X-Rite have ColorTRUE. Both can access your camera roll and offer features like viewing with/without colour management, etc.
So should you bother calibrating your iPad screen? Unfortunately iPad screens have a very small colour gamut – they can’t show nearly as many colours as a computer screen. However you will see a difference particularly images with high colour saturation. If you have a Spyder or ColorMunki give a go and see if it makes a difference for your images.
And yes, these work on iPhones and Android devices too.