Honestly I have this account because I'm typically lazy.
I have a current account with Nationwide, and there is a limit to how much paperwork I want to look at when I get home from work, and how many passwords etc I want to have.
Nationwide offer a reasonable savings account for people who have their other accounts, and I think that will do, at least for the short term particularly while all savings rates are so terribly low anyway.
The online banking is good, which is necessary as the branches are not open at very convenient times, and when you do go there, staff are hard to come by.
Like most banks and building societies, Nationwide seem to try to trick you into getting less good rates by either reducing them after a year, not allowing transfers into the account, or only giving the top rate to people who have particular other accounts, even if they are existing customers. This doesn't make me inclined to give them more custom in the future.
Good online banking
Average savings rates