My dc's primary used to mix every year. I have found mixing, on the whole to be a good thing when it comes to the transition to secondary. When they get to secondary, they may be put into a form class with two boys and two girls from one primary - but NOT necessarily from their class at primary. So when DD went to secondary, she was put into a form class with one other girl from her primary school - who wasn't in her Y6 class, but had been in her Y2 class, and her Y5 class. So she knew her. If they hadn't been mixed, she wouldn't have known anyone in her form class.
However, due to my DS1's year group being very small (single form intake) (so unable to be mixed) , they have started only mixing them at the end of reception, and the end of Y2. I find this not so good, as when it comes to Secondary transfer, DS1 may not be in a class with anyone he knows, and DS2 (currently in Y2), may not have played with / spoken to the people in his form since he was 7yo in Y2, 4 yrs down the line he won't know them at all.
And to top it off, one of the parent governors is making a fuss about them mixing at the end of Y2 even - she's a personal friend, but I really disagree with her on this. I hope she hasn't got her way because then my DS2's class won't have been mixed since the end of reception, and in a 3-form intake, that could be horrible when it comes to secondary transfer.
I have found that when they do mix them, you get a few days whining about not being with their friends when you find out, and about 2 days when they go back in September, but after that, they've made new friends and are perfectly fine again.
I won't find out until the 14th July if they've been mixed or not.