Using an iPad mini as an external display.

Hello one and all, I'm not sure if anyone will be able to help but I thought it was probably worth asking here. My question is pretty simple – are any of the apps designed to use iPads/Phones as external displays actually any good?

I'm a designer and often find myself working on the go with limited screen space on a 15" Macbook, at some point down the line I think I'm going to get a spec upgrade and trade off the screen size on a decent 13". Ultimately I want to use my iPad as a screen for windows aren't the canvas in the tools of the Creative Suite (be it swatches, layers etc) and then have the actual laptop screen real estate for the proper work.

It's a setup to one similar that I use at work where I have the canvas on one screen and the extra windows on my laptop screen and one that I actually quite like.

That's where the fine people of The Verge come in, I've seen many apps that offer the functionality but I just don't know if any of them are any good. I'd quite like to have it connect via USB rather than bluetooth/WiFi, but it's not a big deal.

Cheers for any help!