The Bark Bites Back at 2012-04-23 18:33:44:
My first suggestion would be go to moviebytes.com and do some homework. Readers there can submit report cards on all the various contests, rating things such as whether the contest is actually worthwhile, how professional, any feedback, etc.
As someone who has won a couple of contests, been a numerous finalist, top 2%, top 5%, blah blah blah... I'm not a big fan. The only ones that appear to be really worth anything are Nicholl and Austin Heart and potentially Page - but they get thousands of entries and if you hunt around the net, you can read a number of the winning scripts and some of them will leave you scratching your head.
I tend to find it more beneficial to enter competitions that offer categories based on genres - or competitions that are for specific genres only. But beware, there are a lot of scams out there. 300 competitions? Somebody somewhere is making money off of a number of them. Just do the math.
As for the benefits themselves, I personally entered the contests I have with the intent of using any placement(s) to build up a bit of a credibility when it comes to marketing. Does it help? Well, I honestly haven't really tried to market my stuff on my own... yet. But I would like to think that contest placements and very positive coverage will help to show the material has been vetted.
Aside from that, I've been too busy continuing to learn - though recently, within the last few weeks, I did have a referral to someone at WME based on my competition wins so who knows, maybe someday I'll have something more to report back on.