Well, crunches are useless if your diet and cardio work outs are poor. Not to forget, doing crunches the wrong way. For me, cunches works great to develop abdominals. I couple it with cardio and proper ab diet regimen. :)
But to maintain my abs, not only i do crunches alone, i change my ab routine almost every time i work out. i do crunches, i use the swiss ball, hanging leg raises etc.. I also do variety of crunches to develop different parts of your abdominals.
For example, for lower abdomen, i do the reverse crunches. for the mid section i do the normal abdominal crunches, for your obliques i do the oblique crunches, but mostly decline oblique crunches.
There are other ways to build your abs, if crunches doesn’t work for you try looking into different ways.