Jamie Schissler, Strategy Director at Avenue A | Razorfish, has this to say on the topic:
Having worked in the promotion marketing space, I love sweepstakes. They should be a staple in every brand and marketer's toolbox, and I've seen them executed with tremendous success. But just as you wouldn't use a tape measure to drive a nail, sweepstakes are not particularly effective for database growth and development. As a promotional strategy, they are great; as an acquisition strategy, less so. Let sweepstakes supplement your acquisition activities, not spearhead them.
Always good to see somebody agreeing with me on that subject. I've seen many senders run into many issues by using a sweepstakes as a list building approach. It's definitely not something I personally would recommend.
Switching hats for a moment...in case you're wondering which address(es) of mine started out legit but ended up geting the most spam? It was the address that I gave for a sweepstakes in 2003. Buried in the T&Cs was legalese that said they were allowed to sell my address, and wow, did they ever. This address now gets every kind of spam with every kind of falsity and deception. Bad subject lines. No postal address. No way to unsubscribe, etc. All traced back to this one address that I used in this one place.
As a consumer, this was a huge turnoff, that made me never want to give out an email address for a sweepstakes ever again. Yuck.