As a 'solo quilter', I like challenges because they make me work in new directions. I've only entered two (so far) and they were at opposite ends of the scale...one at my LQS and the Hoffman Challenge.
I think the two parts of any artistic challenge...the interpretation of the theme and the execution of the work...should carry equal weight in determining success. Whether your success is measured by formal judging or by your own critique of your piece, really doesn't matter in the end. Meeting the challenge, any challenge really, results in growth and that's the whole point.