Either are acceptable. I am not familiar with any difference between think of and think about, either in their formal meanings, or in their contextual or regional usage.
I would use think of, but that's merely personal preference.
Does it matter when you ask people about the service in general, and when you ask them about an instance of the service, I mean, a particular occasion?
If you want to get in the nuances of this, you should be more specific in your question, e.g.:
What do you think of the service you have received the last time you stayed at The Glitz-Glamorton Hotel?
vs.
What do you think of the service you have received during your stays at The Glitz-Glamorton Hotel?
Be aware that survey responses (whether you are conducting a formal survey or just asking someone informally of their opinion) are highly influenced by the form of the question. If you want the response to be open-ended, think of or think about are probably good phrases to use. If you are looking for a specific aspect, mention that in your question, and use another phrase besides think of or think about:
How would you rate the service you have received the last time you stayed at The Glitz-Glamorton Hotel?
Were you satisfied with your recent stay at The Glitz-Glamorton Hotel?
Did the staff at The Glitz-Glamorton Hotel meet your needs during your recent stay?
Was the staff at The Glitz-Glamorton Hotel courteous and friendly during your recent stay?
Based on your recent stay at The Glitz-Glamorton Hotel, is there anything you would recommend we try to improve?