We had a look at getting the free-sat stuff about a year ago and came to the conclusion it was more worthwhile to subscribe. You usually end up paying quite a bit for equipment you'd otherwise get free, or even if you get it free you don't get all the channels without a Free-Sat card which seemed fairly expensive for what it is too.
Signing up will get you a free box, installation, more channels and if you put an existing sky customer down as a referer at the moment, both of you get £50 M&S vouchers. Not signing up will get you a permanent free-sat card though and cost less in the long-run, so keep all that in mind when you're evaluating costs.
Freecycle might be a good source of a free box if you want to look, with all the people upgrading to Sky+ now it has no charge there may be a few normal boxes about.