Basically, as a US Citizen, even though I will only be transiting via the shthole of an airport (LHR, obviously), I need this ETA.
The process seemed* painless when described, but is rather painful. Essentially, they WANT you to use the mobile app. They do everything to make that happen (unless you are applying for someone else, in which case you may use your PC/laptop).
So I downloaded the iOS app; you have to take a selfie (so, obviously, as well lit place, neutral background, etc etc). The selfie itself took a few tries. Then you pay GBP 16 (USD ~21).
Then, the worst experience was matching the NFC-enabled US passport with the app, so that it reads the stored info from the passport chip. My US passport is recent (renewed within last 6 months). Try as I might, I just couldn't get the app to "read" the NFC-stored info (on the back cover of the passport). I tried 15 times, with the passport held at various angles, touching the iPhone here and there. It worked on the 16th try (= the passport backcover has to be held EXACTLY halfway down).
"You are holding it wrong" x 10000
I almost gave up half way thru this extremely frustrating @#$@@!!!! experience. Even as I write this I am cursing the app developers.
I can only imagine how somebody else -- say a senior citizen, who may not know tech enough, or whose fingers are not nimble enough, etc -- can easily give up this process after just a couple of tries. The usability experience is just plain shitty. Think about the consequences.
I hope the app developers are reading this.
I'm just glad I dont have to do this for 2 more years.