Where possible (such as when using TFS for bug-tracking and source control) link the checkin directly to an appropriate work-item. Failing that, add the work-item / bug # in the changeset comments. This is the bare minimum acceptable level of changeset comments.
The checkin comment should describe the intent of the change made, only adding details on the way the change achieves the desired results as necessary. A good starting point would be the title of the appropriate work item.
One or two sentences is a good target length for the comment. Don't be so general the comment is meaningless (e.g. "Fixed a bug in xyz"), but don't hide the intent of the change under layers of unnecessary detail either.