I've heard negative things about their online banking and customer service, but those were a while ago so I'm not sure what the state of play is now. Could be worth a try, I do like their ethics.
Particular nits are: - pressing the browser 'back' button triggers an immediate logout - they don't use email for anything except to tell you they've sent you a "secure message" that you have to log in and read - no data export (though there are greasemonkey scripts)
Their customer service I've found to be fine unless you want anything usual, complicated, or to be done in a timely fashion. Much like any large company, really
For example, I can pay in a cheque or withdraw funds over the counter in a Coop Bank branch, but if I want to set up a foreign bank transfer I have to use the an online secure message. When I asked why I was told over and over "smile is an online bank". Which I suppose is true, up to a point.
In lesser annoyances, they make you do a stupid amount of typing to log in, about the only bit of which is actually secure is two randomly selected digits from your 4-digit PIN. All other info is insecure: account number, sort code, first school attended, yada yada yada. All in all they make you enter something like 30 characters spread across 3-5 input boxes and two pages, all for about 6 bits of actual entropy. Unless, of course, you're the sort of person who when asked to tell your bank what your first school was, replies "8EOHzxdO6QnJ".