I guess the other point to make is that a free watch to review would imply that this might not have been something that the reviewer has chosen themselves. This probably has a bearing on the review itself. If you'd seen a watch, considered the alternatives and then decided to purchase it you might have a very different view to if it had arrived in the mail and is effectively not your choice. Reviews of items we have been looking forwards to and loved enough to buy are likely to be quite positive I'd guess, those which are for items which are given for the express purpose of review maybe less so.