I think it is a useful tool; it may make folk a bit more wary and keep to social distancing/ mask wearing guidelines a bit more, which in the supermarket and shops I frequent can be a bit lax.
I think it will also be useful for people you come across who you wouldn’t necessarily have contact details for, that taxi or bus you travelled on, the waitress that served you or the gym you visited.
It may make you more mindful to look out for symptoms in yourself and perhaps curtail your socialising/ daily routine for a wee while (not visit grandparents or meet that friend for coffee etc.) however I can understand this may make some more fearful and cause anxiety.
Yes not everyone will download it and I get that but if this app helps to avoid another national lockdown with resulting closure of businesses, schools and universities then by downloading and acting appropriately, I’m part of the solution: it’s social responsibility.