Having worked in corporate communications for years and done a number of promotional giveaways, including trucks and other high-ticket items... it seems the only thing contests do is attract people who want to enter contests. Their real value is in building a marketing database for later communications.
There are a ton of legals involved in this type of thing too... not really anything to worry about for small ticket stuff (people don't generally spend $10k on lawsuits over $5 trinkets), but the bigger the value, the more likely someone will cry foul. Just be clear on things legally, terms of entering, etc... I think I saw a nolo press (nolo.com) book that covered legal issues surrounding web-based contests and promotions, in part.
I did a web promotion to give away a street sign (I owned it, of course... didn't go out and steal it)... the methodology was to have their entry be a 'post' to a forum, to help get a particular forum moving. I got 400 posts, one each, from people who mostly never came back after the contest ended. Plus, the calibre of posts was along the lines of "Hi. This is my post. Peace--Out!"...
