Some things to consider before you enter one of the bazillion screenwriting contests (and counting) out there.
1. What's their track record?
How long have they been around? Do they meet their deadlines? Provide timely updates? Check out places like moviebytes.com, read the contest report cards, get the skinny from writers on various message boards/blogs.
2. Is your script ready?
I don't know about you, but my first drafts are horrible. While writing to meet a deadline can be an excellent motivational tool, the absolute worst thing you can do is send a script out before it's ready. Give yourself a reasonable amount of time to complete a draft *and* do a thorough polish.
3. What prizes do they offer?
Prizes can vary from year to year and websites aren't always up to date. $10,000 in prizes isn't the same thing as a $10,000 Grand Prize. If it's not clear, ask questions.
4. Do they offer feedback?
A peek into a reader's mind can be valuable, and you're getting something for your money. But honestly, you can join sites like www.zoetrope.com and www.triggerstreet.com, and get free feedback — well, after some peer reviews. There are even folks who give script notes at rates comparable to some entry fees, but if you still want to go down that road... Some contests include feedback with the basic entry fee, while others charge extra. The quality can vary, so ask around....
5. Does this contest carry a lot of weight in the industry?
Not all contests are created equal. If you're just looking to win some cash/prizes, go for whatever looks attractive. Stick to the prominent contests if you're trying to catch the eye of a producer, agent or manager.
6. Is your script a good fit?
A good script can get lost in the shuffle of a big contest. Smaller, niche oriented contests can give it a better opportunity to stand out. Something to consider if your story is faith-based, horror/sci-fi/fantasy, features a female or minority protagonist, etc. A win might not do much for a tentpole script, but a low budget, high concept story might garner some attention from the Direct to DVD and cable-movie types.
Whew! Glad I got that off my chest. Hope it was helpful!