I recently switched away from Starling. The reasons why are that the app was good, but I always struggled with the navigation on it. It didn't flow as well as it could.
Mainly, however, I left because I found the customer services less than friendly, they came across as cold when I dealt with an issue with a Direct Debit being set up. I got the impression that the person dealing with the issue was almost trying to be awkward since they refused to talk to the organisation trying to set up the mandate, and I had to do it myself. It left a bitter taste with me, and as such decided to leave.

In short, good bank, let down by the app navigation and customer services.