Adding comments to user profiles I would like users on my site to write "feedback" of other users.

My thought is to have a content type called Feedback, and create a view of it, to preview at the foot of the user page. I can also put the main display of comments on its own views page.

Well, that's the display of them covered, but how do I enable them to be written? ie, link from the profile to create Feedback, and have it relate to that profile.

Also, Profile2 looks lacking, because you cannot control URL output.
You can use the entity reference field. Entity Reference module provides the autocomplete field for the user reference.

Add an entity reference field on the feedback content type. This field is autocomplete field. So whenever you create a feedback you can select the name of the user for which you want to add the feedback and refer the same.

Still if you are unclear then provide me some more info so i can help you out..

Update: In your case you have to create a link on the user profile page and add the name of the user being viewed in the query string. So if you are viewing the profile of user1 then on the user profile you can generate the link $base_url/node/add/feedback?name = user1

Now when any one clicks on that page you will be redirected on the node creation page and you can get the argument value from the URL. You can set this argument value in the form field using hook_form_alter() hook.

Update 2: I found something like your question here