I bought it from day one and I did not regret it.
Despite on this moment it is still in beta, I can use it and build up my knowledge about Construct 3. That is on this moment the advantage I have.
Anyway in its present state, Construct 3 is very usefull. I came accross some small things which did not work as expected, however the majority of features works very stable.
Personally speaking, I am happy that I bought Construct 3 from day one.