Monzo is something I've been keeping an eye on for a while, and having been at the top of the waiting list all summer, I was straight off the mark when they finally launched on Android on Thursday.
At the moment, they're providing pre-paid debit cards (chip and PIN + contactless) with a really nice app and an API. I handed over my initial £100 top-up late Thursday, they posted my card on Friday (notifying me through the app, naturally) and it arrived today: so far, so efficient. The fact that these are non-personalised pre-paid debit cards helps, as they don't need to stamp your name onto the card: they can just take one off their pile, note the numbers and post it.
The card is a loud orange/pink colour, but that's more than made up for by reliably instant notification on my phone (and watch) of transactions being made. As others have blogged, the phone often buzzes with the notification before the retailer has even handed the card back to you. As someone increasingly frustrated by the way some transactions take 3-4 days to show up on the app for my credit or debit card, this is much more like it and appropriately 21st century.
They also don't charge extra fees for usage abroad; they simply pass on the MasterCard exchange rate. This is another serious annoyance of both my credit and debit cards, and something I look forward to trying out when I travel to ApacheCon next month.
The API also promises the possibility of being able to securely and in near-real time pull details of payments into my budgeting app instead of having to wade through a pile of receipts and enter it all by hand at the end of each week. I haven't tried that yet, but will report back.
They're aiming to get a banking licence and run a full current account in this style - a delicious prospect, though I hope they're thinking about the details as I can imagine instant notifications of every standing order and direct debit on the first of the month being information overload.