Been using them for a couple of years, as has the wife. I'm a big fan - the usability and flexibility of the app and accounts are top notch and are definitely a more modern way of running a current account. We also looked at Monzo and Revolut, who are similar.
We've not had any issues so haven't had experience one way or the other as to what it's like when things go wrong, but they were great when setting up the accounts.
The Trustpilot reviews for Starling seemed on the whole pretty good to me, certainly a lot better than the big banks:
https://uk.trustpilot.com/review/starlingbank.comCould it just be a case of people complaining when things don't go well, but not saying much if they do?