It is better to ask for comments only in the pilot study that precedes a system-wide survey Here such remarks can be useful in making the final instrument more comprehensible and clear. Surveys set expectations for change. The act of asking for people's input raises expectations that something will be done with the results. Training participants often ask later to hear how the staff used their ratings and suggestions. Be wary of asking about things that are not going to change.
Surveys are quick, taken at a time when people want to leave.
Many training participants approach the task of evaluating a course in a cursory manner. After all, it is not their course. Statistical trends depend as much on group composition as on design and delivery. Statistical trends are not comparable when design and delivery change. Not all instrument items are equally important, and often some of them do not even match the rating scale.