Last minute second guessing is normal. If you loved this name when you decided on it, have faith in your decision.
It is a bit plain, but that can be refreshing in a world of Zaidyns. For me, it would depend on family significance and the last name. Smith? Maybe too generic. Walenciewicz? An understandable nod to balance.
I agree with all this. John was one of two finalists for DS. I liked it because of the family significance (it's DH's grandfather's name), the biblical figures named John, and the strength and simplicity of the name. Ultimately I'm glad we went with our other choice - one negative for us is that we have a common, short last name, so polysyllabic, more unusual first names work better for us - but John is a classic and I wouldn't discourage you from using it if it's something you've already committed to. And I can tell you, since DH and I didn't commit to either child's name until after they were born, some second-guessing is inevitable.