I generally hate them because they are usually not "design" contest at all.
Design is a process between a client and the designer where parameters are set up and some amount of thought is put into it while going back and forth to perfect it. A contest is usually a beauty contest where you have to guess what you are being judged on. It usually ends up mostly with a cookie cutter look or as nebulous as the contest rules may be. I have seen good work on some by very talented designers, but my firm belief is that it would be a lot better if they followed the design process.
Most
sign companies engage in the same process when clients ask many shops to design a
sign.