I am hitting a handful of competitions this year. I finally think I am ready, but fully expect not to win anything. As Ezzie said, you can't be sure what the judges are after any given year and looking back over past winners it is clear that some competitions/agencies try to mix things up for the sake of diversity, which is understandable.
You have to give it a shot if you think you might have a chance. You just never know, although mentally I will be forgetting about them as soon as the submission has been made.
The most exciting thing about these competitions is not the competition, it is the process of preparation. For the first time in ten years I feel I have defined my photographic groove. Despite shooting landscapes and feeling close, it took a shift towards documentary work to finally click. That might sound odd, but put the landscapes and the docu work together and you would likely see the parallel.
Working towards structured goals has improved my work no end, improved my editing, my ability to articulate what my work is about and if anything has made me more passionate than the already uber obsessed state I was in. Even if I win nothing, that wont change. It has been truly good for me and the future will hopefully bring more exciting projects and personal development.