I've done this a few times. I'll sign up for a free (or low-cost) trial of X with full intention of taking whatever teaser offer they have at the time, and then canceling.
Last night I signed up with an internet fax service. It has a 7-day free trial. I sent a 36-page fax out with it. Even though the service is only $7.99 per month (for ten times the fax pages I sent out) I still can cancel and not pay a dime, even though their tech support worked with me for about a half-hour to ensure that my fax got sent correctly. My other option would have been to drive out to Kinko's and pay $1 per page to get this thing out the door.
So I really do feel that I owe them at least a month's service.
Should this be an issue? Or am I just one of the smart ones that got something of substantial value for free, and everyone else is a sucker?

