I laid a complaint about the Johnsonville store a few months back.
Often orders would take forever because there wasn't enough staff - I tolerated it.
Went there on a Saturday, food all over the kitchen floor, one guy had a fry stuck to his shoe - I tolerated it.
The next day I went back, ordered a MegaMac combo (it's a BigMac with extra meat) but they didn't give me the free glass (they had in the past, because it's basically a BigMac which was in the list) so I asked politely - but I wasn't happy about his attitude towards it (I wasn't being an ass, I promise) and there was no way he was going to part with a glass (even though it costs like 20 cents to make probably). It was a bit bollocks because if I ordered a BigMac with extra meat I would've got it and had the same damn burger.
So I wrote in and complained about all of it. I got a call from the manager the next day, a written apology and a $20 voucher.
After that for quite a while the store was much better staffed, clean, and annoying customer service guy was no where near the tills anymore.
The franchise owners / managers freak out when McD HQ gets a serious complaint - so it's actually worth the time to have a good old rant
I just used their website, so didn't have any dramas with crybaby teenagers.