We are fairly new to keeping parrots, and so did lots of research before choosing a bird - almost everyone suggested that Amazons can make the best 'companions' of all the parrots because they are intellegent, funny, and generally very good at mimickry - true, they can also be incredibly independent and strong-willed, but that is also what makes them robust and more 'forgiving' of your occassional mistakes.
Of the problems we were warned of, it was that they can be noisy, that you really have to give a balanced diet (and watch their weight!) - and that sometimes their 'independence' can tip over into agression (especially with males in breeding time). I know you wanted the possitives, but every species has their individual traits and potential for 'problems' - but it's knowing them and acting accordingly that helps you keep those problems from developing. As to the agression - it's one reason we were recommended to get a DNA sexed female.
As the Capt'n says - having a happy Amazon is mostly about proper care and training, and sensitive regular handling. I'd also say it's about respecting them as a companion and a member of the family - and not expecting them to be a cudly pet

But then, I'm not sure that couldn't be said for having *any* kind of happy parrot.
I can certainly say that Isis has proved herself to be very chatty, impossibly cheerful, often cheeky, fiercely independent and incredibly intellegent - she's not much for cuddles, but she loves head scratches, is tolerant of being touched and positively *demands* to be included in whatever we are up to. And the more we involve her and allow her to be part of our 'flock', the happier and more relaxed she is.
True, she can be a little green dragon at times - but we are
very happy we decided to share our home with her.