It's printed on every receipt and literally takes five minutes to do but it's so helpful. The restaurant gets to read the comments and it can lead to the staff getting a fairly decent bonus. In return you'll get a big mac/chicken sarnie/veggie and chips for £1.99.
I know it's a bit of a pain but depending on the restaurant it can be part of someone's job (customer experience leader) to get this feedback and if you say you'll do it and ten don't they tend to get moaned at for not getting enough. (The cel in our store is fantastic but I've seen people agree to do it to her face then chuck the reciept in the bin which seems a bit mean spirited)