Have you ever been offered free products in exchange for a review on your forum? Did you accept them, or feel it was unethical?
I'd feel it was unethical but If they were good freebies sure i'd take em unethical is different guilt, guilt you live with unethical you just deal with.
We do actual product reviews. I contact various vendors in the space we are in and request product to review. They ship to my review staff, we install and live with the product for 30 days then uninstall and send back. Write the review, send to the vendor to add comments that are then posted as a sidebar in the review. The vendor does not have be a site sponsor to take advantage of this service.
We do regular product reviews and the person(s) reviewing them often get to keep them as the manufacturers often can't sell them once they're used. It's worth it for them too, so it's not unethical. The members get to see a review, the supplier gets to get their good product reviewed, and the person taking time off work to review them (or in their spare time, even worse IMO) may sometimes be lucky enough to keep the odd item. Above all, the review and the supplier will get unspoken kudos granted from the members for creating something to read and talk about.
I've never had such an offer. But we do have a Product Reviews section where members can review (positively or negatively) products they have used.