I don't think it ever hurts to give companies feedback, both good and bad. When I was in sales I used to tell all my customers that if they weren't happy to please let me know. If I don't know you're dissatisfied, I can't rectify the problem. Unfortunately, there are jerks around who just complain constantly or are always angling for something free, but thankfully there are relatively few of these sociopaths around.
I bought a Dyson vacuum off Craigslist and it was terrible from almost the first day. The person obviously abused the machine, which was apparent from the first time I opened it up to try resolving a problem. But after finding a couple of HEPA filters on what was billed as a "filterless vacuum," I contacted Dyson to question how they could make this filterless claim. I told them about my experience at the same time.
I told them in my email that this was the worst vacuum I had ever owned and would take a much cheaper Shark in exchange for my current machine any day. Dyson was in touch with me almost immediately to talk about the issues I was having. They ended up REPLACING my vacuum (an $800 product!), even though I told them repeatedly that that was completely unnecessary since I had bought the thing secondhand. Their response was "We don't care. We don't like it when people don't like our vacuums."
And that's where the spreading good stories comes in. I think we are quick to spread bad feedback about companies, but don't do the same for good stories. I try telling this Dyson story as often as I can just because good companies should be rewarded for being good.