Me!!!!
Known as 'The Peter Pan' of the dog world. My father worked and trialled them and had the odd litter so grew up with them. We've chosen to keep one ever since so about 35 years living with them!
They are, in general, much livelier than say a working lab or golden and less easy to train/handle than the other retriever breeds. They love life and are full of boundless energy and enthusiasm. It is said not to own one unless you have a good sense of humour or if you object to guests being kissed all over. They are very affectionate clowns. In general they adore children, adults and all species of animal. They can be constant playmates for children. They are always ready for fun/silliness.
We have an old boy at the moment who is 10 but people are amazed when we tell them his age. He still behaves like a pup! My children think the world of him. Mine have grown up with Flats but some children maybe 'afraid' of the Flats pushy, boisterous nature. However, my Flats have never even accidentally hurt the children and I've been able to trust them as much as you could trust any dog. They are in general very tolerant of small children and there is not a bad bone in my dogs body. He is, like most Flats, an amiable fool.
They can be a bit hyper and obsessive. They need lots of exercise, lots of attention and something to do .... especially young males. I did a bit of agility with my current dog plus trained him as a gundog. Never intended to go shooting with him but it helps to get the level of obedience that makes them easier to handle. Young males often need 'bossing' to stop them getting out of hand. They are in general, no where near as easy to train as a working lab and not as easy to live with. They are not as laid back. They are constantly 'on the go'' and tend not to know when enough is enough.
Flats can be very boisterous, particularly the males. Several people have come to fuss our Flat and said that though they loved them they had parted with theirs (all males) because they couldn't handle them. The males can be strong-headed and do need firm boundaries set. They can be hard to handle 'physically' so need to be trained to a good level. They are big characters with big personalities!
In trials/working tests, not many, if any, can honestly compete with the good old working lab! If you are interested in that side I can run through the differences as I see them.
I did once own a Flat who was aggressive to people and fought like a street fighter. I was only a teenager but trained and worked him and went picking up with him. The gamekeeper wanted to buy him 'an old fashioned keepers dog' he said. He would have kept you safe from poachers, I guess but would have been big trouble had he not been so obedient. He was a one woman dog but totally loyal to me. I know people always say 'check the parents' but his were fine and had the more usual Flattie temperament. So just to say .... they do vary as with any breed. The breeder of my current Flat says that she sees a bit of 'dog aggression' in the breed at shows. When they do fight they can be surprisingly good at it.
Cancer is a problem in all lines of the breed. They originate from a relatively small gene pool of dogs remaining after they fell from popularity as the main retriever breed before the war. It is a worry for the breed as a whole. My sister lost one at 1 year old and I remember being heartbroken losing one at 7 when I was a teenager ..... my first working dog. You will hear lots of sad stories but having said that we have been fairly lucky with them and had on until 14. one 13, one 12, one 11 and one 10. Latest lad is now 10. I'm still in touch with his breeder. His mum is still alive and all but one of his siblings.
I love them

. Have tried to mention the good and the bad points. Not as easy as the other retriever breeds but HUGE personalities, loads of FUN, loads of ENERGY, BUSY,BUSY BUSY. Not a sit in the corner type dog but if you are that way inclined they will make you SMILE.

HTH