I understand the need to spread the word, etc... and you are giving out free stuff.. but forcing people to follow and retweet your message, without even giving the option to edit the text.. I don't like it.
The act of forcing someone to retweet a message could be reasonably construed to violate the consideration clause in Federal and State sweepstakes laws.
In addition tweeting a message that appears to be an endorsement without disclosing that you received something of value in return might technically put you in violation of FTC rules.
what happens when non marketers market--see we know this kind of stuff. :) Also, just because a company is in the "pack" doesn't mean it's right for your startup. Many of these products are way too expensive to keep for more than 90 days; and the switching costs of taking them out are inappropriate for the value given.
I don't know about where they are based, but it is certainly against relevant legislation in the UK (that is why you always see "no purchase necessary" text and an alternate participation method ("send a stamped, addressed envelope...") listed on "competitions" like this.
http://contests.about.com/od/c/g/consideration.htm
Any sweepstakes that requires consideration from an entrant is illegal.