Things aren't perfect, I never would say they were....however.
Guaranteed Bag
There are some folk who just don't want the 'hassle' of having their bag placed in the hold. On very busy flights (very dependent on route and time) it's not/wasn't always possible to accommodate everyones bags. I guess Easyjet could have charged for cabin baggage or enforced a reduced dimension bag. Instead what they say to people is, if you want your bag guaranteed to be kept with you in the cabin on busier flights then you should adopt the guaranteed bag dimensions. That way it will fit under the seat and it will always fly with you. The bag size is bigger than you think.
Even with the bigger bag it the chances of you being parted is fairly slim and in many ways it is just like unallocated seating. Only the last few passengers on very busy flights got sat apart.
N.B if passengers are considerate when stowing carry on baggage then that helps a lot. i.e place coats on top of bags, place carrier bags under the seat, place bags wheels in first etc etc.
Allocated Seating Vs Turnarounds
Easyjet's turn times and OTP is still right up there. I've not noticed any real effect on turnaround times. Some crew, notably those who had only ever known Easyjet, didn't particularly like allocated seating to start with simply because they knew nothing but free seating. And there is sometimes an element of protectionism of the 'old Easyjet' among those who have been there for years.
Customer satisfaction has increased immensely with allocated seating and apart from the odd question about seat locations I really can't tell any difference when boarding on a day to day basis.
Customer Relations
I can't comment on specific experiences but I can assure that feedback about crew, both positive and negative, gets fed back to crew including individuals. Make no mistake about that. You can also e-mail Carolyn direct. Inevitably there will be someone screening the e-mails i'm sure but I know people who have had personal responses and she often comments via her communications about specific feedback she has received.