I took up with the iPad as soon as it came out. I began to do demos with it, and champion the device as a useful addition to anyones' DJ set-up. I often read comments that implied (and often blatantly stated) that the iPad was a "gimmick" and not truly useful.
At the time, I couldn't be sure of where the iPad would take the art of DJing, and subsequently the development of DJ gear, but I felt strongly that it would play a significant role in both of those areas. I just finished watching a review of the new Denon controller, in which the significance of the line inputs was downplayed. It would still seem that the idea of running an iPad into a 2-deck controller is just a bit above the heads of many.
What is my point?
This...the reason you want to adopt new tech as soon as you see "possibilities" or "opportunities" or "capabilities" is so that you will reach the point of truly understanding the proper implementation first, before your competitors. DJing is a performance art, is it not, it is still entertainment, and to be honest, modern DJing probably looks like science fiction to many of the uninitiated.
To sum up,..an 'iPad running a good DJ app' output into the line in of a 'two deck controller running good DJ software' gives you one heck of a 4-deck system, yet with the versatility to be a small portable 2-deck system or even smaller and more portable iPad system when needed.
And here's the kicker, I came to this knowledge by believing in the iPad before I could see truly where it would fit in.