I saw it a couple of days ago and it's - you know - OK-ish. I think the reason it is doing so well is that it is a little bit scary and atmospheric, but not disturbing or challenging. Also it's a 12 certificate so it's as near to family-friendly as a horror film can be. The horror factor is almost entirely supplied by
And there's nothing to take away with you, nothing to trouble your mind once the film is done. Keep away from sinister haunted houses with grim legends attached to them and you should be OK. The take-away message is probably more comforting than disturbing.
Radcliffe is also OK-ish. He seems like a decent chap, doing his best. I mean both the character and the actor. Someone like (say) Martin Freeman would have made this unbearable poignant. I think Radcliffe's lack of power gentles the film and makes it more user-friendly.
In conclusion, I think this is a horror film which has pulled its punches, and contained its horror in safe bounds, and probably that's why it is doing so well.