I am looking into replacing my iMac for a retina 15. I need to be a bit more mobile and an iPad just doesn't cut it. I do a good amount of design and less so programming. I don't have an external in my budget right now. Nor do I really want one. I am hoping to be able to do what I need with just the 15. Anyone here use it for this successfully? Can you please share your experiences? Thank you!