I've never had an iPhone before, but am making the switch. I pre-preordered (i.e selected what I wanted, a 15 Pro), and had that ready to go in my account when it went live (13:02 in the UK). Checkout process was pretty quick, just needed to sign in with my Apple ID and then confirm a couple of details. Was done by 13:04. Supposedly have a delivery date of 22nd September, but we'll see about that! A few days won't matter ironically as I'll be at PyConUK, but here's hoping.