Lots of advantages to this model. You'll bring in more revenue, sooner, and implicitly get the customers commitment for the year.
On the downside, you'll be giving up revenue from your most loyal and committed customers who were planning on staying with you for the long term with or without the discount.
Optimising this balance is, of course, the challenge!
Re level of discount. One thing which will influence this is the customers likelihood of attrition. On a product such as a CRM system, once they are committed, they will likely be around for the long term due to cost of migrating away. For a business such as RapidShare, people come and go quickly and with limited cost to them.
I would always tend towards using these kind of disounts, though would be more inclined to heavily discount in the RapidShare type of business.