I used to have a pair of sofas that had come with a 'dress' pattern, so I made new covers a couple of times. It wasnt difficult, because of the pattern, but I imagine making your own would be more tricky (or maybe not, just pin and sew in a logical system)
But it took ages, I took a week off work, so at that rate I didnt save too much money!
Our present sofas are a more complicated shape and I have even less time now, so for this year at least, I am convincing myself that 'shabby-chic' is a good look!