Simple cocktail, and I thought it was very good. I enjoy Negroni variants - I think this is quite a distance from a Negroni though. Brilliant, clear ruby color, with a hint of yellow/orange - nice in the glass. Rum character leads, but it is closely followed and bracketed by modest bitterness from the Byrrh and Campari. The rum, Byrrh and Campari also work together to create an interesting aged spice flavor. Not a favorite, but worth a shot for those looking to try every possible wrinkle on the Negroni template.
Tried this tonight and was underwhelmed. I enjoy bitter, but I found this more than I cared for. Will give it another go with slightly increased Dubonnet and less Campari. Depending on how that goes may try swapping out the Campari for Aperol.