I had a bad case of seasonal flu a couple years ago. I started feeling lousy one evening and then the next morning woke up with a fever well over 103* and the whole gamut of symptoms; body aches, chills, headache, cough, sore throat, etc. I was still pretty sick but NOT like that first day- the fever came down a couple degrees which made it much easier to control with Motrin/Tylenol and the symptoms eased a little at least. It wasn't a miracle worker but it clearly helped. I count that flu as one of the worst illnesses I've had in a long time and I can't imagine how it would have been if it stayed as bad as it started for the entire acute phase of the illness.
However- despite the Tamiflu, I still developed a secondary bronchitis. So I would still be really careful about keeping tabs on our kids even if they seem to be recovering quickly w/ no lingering cough. We do that anyway of course...
Oh, and my dr had stressed the timing aspect too. Since I was in to see him the day I got acutely sick and less than 24hrs after my symptoms first began, I was in the best window of time for the Tamiflu to work most effectively. Not waiting is very important.