I feel like the connotations are too strong in this name to use it for a child. It's very hard not to associate it with something like a stoic judge or king, so it would be jarring if a Regulus grew up to be a free spirit or energetic daredevil. And it wouldn't be very nice to force a child to grow up a certain way for a certain role.
I love this name. Not just because of Harry Potter (and I think only true fans know it is from Harry Potter so there is not too much association), but it sounds strong. Regulus, the character, is strong and an unsung hero.
This name has grown on me considerably after re-reading Harry Potter and the Deathly Hallows followed by (I'm sure) hundreds of Marauder era / pre-Azkaban fanfics. The character of Regulus Black (although we don't know much about him canonically) gives the name a strong feeling of cunning and bravery. If I were to name my child Regulus, I would do so hoping that he would grow to be very intelligent and ambitious.