It's currently only possible to search by "reviewer" (or group of reviewers), which actually returns "Changes that have been, or need to be, reviewed".
We need a search operator to get a more useful result, without that "or need be".
As +1/-1 etc. reviews are actually canceled by new patchsets, it could be called commenter:, meaning we look for change sets with at least a comment by the user (whether about the last patchset or not).
Note: issue 1248 is different, requesting an operator for the Author field to find also patchset authors.