I've been through the process twice, and in neither case was there any pressure to sign the document. In both cases I was advised to speak to a lawyer if I was unsure or uncertain about anything, to feel free to contact them with any clarifications and in one case I expressly forbidden from signing it in the HR office and told to mail it back.

A lawyer would tell you that such a clause is common, is somewhat enforceable, and rarely enforced. The situation primarily being avoided is airing specific, proprietary, disparaging information.