I've also noticed in the past that there are differences between accessing Apple's on line sales system (the UK site in my case) depending on whether I'm doing it using my PC's web browser or the Apple Store app on my iPad. Last year for instance I was monitoring the UK buyers thread here and at about 1pm the length of the thread grew by about 4 pages in a single minute with a load of "It's live", "I'm in", "Order placed" etc messages whereas I was frantically hitting refresh for another 10 minutes and was still getting the "We'll be back soon" message (or whatever the exact message was). I then went to the other device and got straight in. I can't remember which way round it was, my PC not getting in and when I switched to the iPad I was OK or the other way round, but I definitely missed about the first 10 minutes of the store opening due to some issue on one of my devices.
What I've done this time is save a pre-order via both devices and interestingly in doing that I noticed the two systems don't seem to be fully tied together. On Tuesday I already did a pre-pre-order (the setup for the pre-order I want to make tomorrow) via my PC web browser but when I checked yesterday using the Apple Store app on my iPad it wasn't there so I set up another pre-pre-order via the app. I just logged onto my Apple Store account now via my PC and now I see 2 pre-pre-orders, the one I did on Tuesday via my PC and the one I did last night via the app on my iPad. I intend to leave both active and tomorrow I'll be at my PC with my iPad next to me and I'll complete whichever one I can get through the system first and then cancel the other.
Also in the past I have sometimes had early attempts at getting an order through lock up or even give a failure message so I've tried again, sometimes a few times, and then a few hours later after all the intial craziness is over it's turned out that at least one of the seemingly failed attempts succeeded so I actually have 2 orders in the system. That's no big deal since Apple has great cancellation and/or returns policies but maybe something to be aware of.
In fairness to Apple I have found the systems to be a lot more reliable in recent years vs say 5 or more years ago. In those days there could be lots of freezes and transactions timing out in the first few minutes after pre-orders opened and it could take multiple attempts to actually get all the way through from selecting your phone to getting payment accepted and order confirmed. Apple's ordering infrastructure does now seem far more able than it was to cope with the huge spike in orders immediately after the shop comes on line than it was a few years ago without so many repeated failed transactions but you do still need to be quick if you want to get one of the more popular models for launch day delivery or collection.
One final bit of advice alluded to above but I'll make it explicit. I do always keep a tab open on the "Pre-release, Orders and Dispatch" thread on this forum (in my case the UK one) so that I can see when others see the web store open for business. If it wasn't for that then last year I might have missed out altogether when I couldn't get in via whatever method I first tried and might have just thought it was some delay on Apple's side and kept on trying for another 10 or 20 minutes while everyone else in the UK was snapping up all the launch-day stock.