hmm, anything that highlights the good parts of the artist's service and artwork should suffice.
- did the artist present him/herself in a professional and courteous manner? were they ever rude to you or other customers?
- did the artist give you the artwork in a timely manner?
- was the artwork the same level of quality that you expected?
- was there a good amount of communication between the two of you?
- was the price appropriate for the quality of their artwork?
you can talk about anything in regards to their service, to be honest