We just returned from 15 days on the Bliss where we were in Haven and had the Premium Plus package. I agree with the assessment of @sthrngary and others that the P+ package really shows its true colours in Haven. Our experience was that the Haven bartenders were extremely knowledgeable, and made some fantastic cocktails if given a bit of direction as to preference, and then allowed to experiment. I was introduced to a number of cocktails and high-end spirits that I would never have thought to try otherwise. For us, it was well worth the money.
Having said that, I also found that the staff at the A-List bar especially had access to a very good selection of spirits, and would ensure that our cocktails were made with high quality ones when they saw that we had the Premium Plus package. At places like the Atrium bar, where we generally ordered simple drinks because of how busy it was, I would often ask for them to include the highest quality spirit they had (e.g: if a gin and tonic, then the highest quality gin). I also found that at many bars around the ship, even if they didn't have a certain wine or spirit listed on their menu, they might have it if asked (e.g: Atrium bar only has prosecco on its menu, but had a bottle of Veuve available when I ordered bubbles, ymmv). Part of it, for me, was realizing what bars catered to what an ordering accordingly (I certainly don't expect the bar at H2O to have a bottle of Chateau Neuf du Pape on hand, for example).