The following is my experience, others may have wildly different experiences:
In all honesty it's been very stressful, for the family and for especially my DS.
He's a bright boy, but quite resistant to being taught outside school - certainly by members of his family! His school is not really equipped (through no fault of their own, just a part of the system) to identify and have the time to push boys who might show promise but need inspiration.
If it's clear that your child is truely exceptional it's easier for state primaries to recognise and encourage (not always successfully, but at least the attempt is made), it's the ones that need inspiration to get the best out of them that get lost in the pack a bit.
This may not be the same at all state primaries mind you, but is certainly true of ours, and this is a relatively high achieving one with a good mix of kids and parental ambitions.
I, mistakenly, thought to do the preparation myself to get DS through the process. This, in hindsight, was a horrible mistake. Unless you have a very grown-up relationship with your 10 year-old it is difficult not make things very stressful. Primarily, because it continued the stressful parent-teacher relationship that started with lockdown, but also made it very, very hard to judge his ability level compared to his peers. With a tutor to help prepare, it may have an added cost, but this comes with the vital additional benefits of, for example: knowing where your DC sits among peers, the idiosyncracies of each school's process and, most importantly, protecting your relationship with your child.
Now we are at a final stage, the interview, which I have only the vaguest idea how to prep for! It feels like a final hurdle in a long journey, that still may end in disappointment. It does feel like an acheivement for DS to get to this point and am very proud of him, just no less nervous than I was at the start of the testing period - and that state of nerves is truely visceral.