I'd personally go with a company who was installing insulation BEFORE this free deal 'cos then you know they are a company who specialises in this rather than some dodgy taking advantage of this situation/offer IYKWIM. You should also be able to get "name brand" insulation ie Bradford or Pink Batts etc so you know you are getting a quality product.

We got insulation 6months before this offer but did get a partial rebate - we used the yellow pages to find a company.