Diaries can help users to express their impression after single testing moments for example when testing repetivitive services over a longer timespan like teaching modules. Ask them to write down what they appreciated, what was difficult and what they noticed in the particular moment. Writing it down immediately cathces the impressions while they are are still fresh and unbiased by internal confrontations or opinions of other participants.
Requesting a diary entry after every session/element of the testing will give you the opportunity to obtain a more diffused view on your service – are all sessions perceived positively/negatively in the same way? Are there some that are appreciated in particular and why?