In the music production the iPad is still regarded by most as a toy rather than a tool. Some believe that it can not give a good enough quality of sound. Others believe that there is no quality sound processing applications for iPad. All this is not so.
What you need to use iPad in the studio or in a live performance? Not so much.
iPad (I use iPad 4 with Lightning connector).
Lightning to USB Camera Adapter (for iPads with Lightning connector).
External high quality audio interface.
In fact built-in AD/DA in iPad can't give a good enough sound quality for studio use. But can sounds pretty good as example for a live performance.
I use Focusrite Scarlett 6i6 as external interface because it has a good value for money. The whole Scarlett series have a very decent stuffing inside:
Focusrite Scarlett 2i4 inside
There are many audio interfaces compatible with iPad. Many of them have their own AD/DA inside. But not all is highest quality of course. Before buy, look at their specification and read reviews. Here is large list of interfaces compatible with iPad (almost all with AD/DA inside):
Audio Interface Compatibility
With regard to the sound processing applications. There are a lot but not all of them provide a high quality audio processing too. The most important thing in these apps (besides of comfortable User Interface of course) it's what the processing algorithms were used.
If algorithms were created with understanding of Psychoacoustics and experiences of work with real analog devices and instruments, iPad can process audio not worse than a well-known professional VST plug-ins or even standalone digital processors.
In all of my applications I use special designed audio processing algorithms. It is unique algorithms were created especially for these applications in cooperation with a professional sound engineer Andrew Startsev who developing and producing audio devices and software for audio recording and mastering for many years.
For post processing I would suggest using my application Audio Mastering for iPad. It the most natural sounding application for finalize your tracks.
[ More about Audio Mastering ]
[ Audio Mastering technical specification ]
For real time processing synthesizer, guitar or voice I would recommend use Master FX for iPad. Besides the wide range of high-quality and creative effects this application has very low latency time what is very important for use in a live performance.
[ More about Master FX ]
[ Master FX technical specification ]