Charlie Jane Anders, over at io9, provides a helpful list of ways to tell if the first draft of your novel is worth salvaging.
If you decide that in fact it isn’t worth revising, don’t despair. First, just by finishing it you’ve accomplished something that most people never do. Second, you’ll have gained some insight into your artistic obsessions and thematic concerns (“Hello, traumatic high school experience– nice to see you showing up again in yet another story!”) . And finally, it all counts for practice, like the scales and Hanon studies and Czerny études that a concert pianist does before he can play at Carnegie Hall.