Hi, I downloaded the BT SmartTalk app some months ago. Activation was straightforward.
I haven't yet made any calls using the app but have taken the view that as it's free I might as well have it available to use as and when required particularly if I ever need to make 0800/0845 calls or calls from abroad.
There are a number of independent reviews/write-ups to be found if you google BT SmartTalk, all of which echo the 'why not, it's free' attitude ...