Are you as likely to send a comment to a corporate office for an employee providing you with excellent or do you just comment on bad service?
I do both. When an employee goes beyond what I expect, I let corporate know about this person by name. Hopefully this gets that person some recognition.
If I get poor service, I also let corporate know but never name the person. I just list time of day and maybe department.
I'm kind of a wimp IRL, so if I received poor service I'd probably just make a mental note to avoid that company in the future. (Or return, for good service. Vote with your wallet and all.) Although if it's very bad I am not above telling the employee exactly why they're incompetent and then walking out. Telling corporate you've had bad service without naming the individual is likely worthless since they won't be able to do anything specific about it.
I generally don't communicate with the higher-ups in either incident, because I honestly believe that in most instances saying something isn't going to change a bloody thing, whether it was poor service or excellent service. It will, however, influence my decision to continue patronizing the establishment, whether it be some big box retailer or a local restaurant.
The only voice you can speak with is your wallet.
Yeah, I too rarely comment on anything self-motivatedly. I only give feedback if asked; many times, the Customer Service representative I speak to asks me whether I liked the service; that's probably because the call was being recorded or something.