I made the semi-finals a few years ago with the first script I ever wrote. The dude in charge at the time hand-wrote a note on the cover page saying that he wanted it to go to the finals, but was overruled. Very encouraging! I then got several calls from producers about the script, one of whom optioned it twice. It never got made, but I made a bit of money, made contacts, and got tons of encouragement. I do recommend submitting if you've got a really good script and money to enter.
But, as others have said, do take a look at previous winners and see if your script is of that type. The Nicholls are unlikely to favor a gross-out comedy or a horror film, for example.