I do not have any direct experience with WooBox. I did run a promo once on my FB page using WooBox but I had a company managing my business FB page at the time so no direct involvement in setup, etc.

However, I do have experience with ViralSweep. I have found it to be a good feature rich platform. If you are shopping around I have also heard Contest Domination is rather good.

Sweepstakes and Contest in general are complicated. It takes some time and marketing to get it humming. And, your prize(s) will have an effect too. Obviously, if your prize is a new shiny iPad you will probably get tons of entries. But, are they the entries you are looking for?

I currently have a sweepstakes running using ViralSweep.

Oh, I almost forgot ViralSweep does have a guide with a lot of information about sweepstakes for more information: http://www.viralsweep.com/advanced-sweepstakes-guide/

BTW, I am not affiliated with ViralSweep I just have the most experience with them as a user.