Knowing you are at a point is only necessary to know that this is the correct place to turn left, or whatever. There are no stamps, and not necessarily marshals at every point (a few of us will be dashing round to the main ones). Voice instructions do seem to need sign-up on every app I've looked at.
The walk attracts all ages. For some, signing up won't be at all familiar, and installing an app may be something of a novelty. For the younger ones, if they can get more by signing up, and choose to do so, then great - but, from what I've seen, to get full service you typically need to pay as well. So I really wanted an app that gives some basic service without sign-ins/payment.
RWGPS remains front runner by some distance. I tested it today on a trivial walk around town, and it gave as much as I think I can expect. Pity you don't get a marker to say which cue card you're one - so, you've a list saying turn right, then left, then right, and it's easy to lose track of which instruction is next. But the map is there with a marker showing where you are so, as long as you can look at a map and see that you need to turn right at your current location, it should be OK.