We've done 4, 2 with Cunard, 1 with Royal Caribbean and a Celebrity one. We enjoyed the Cunard ones the best, yes it's formal but I don't mind that, much better than the hybrid you find on some cruises where some people are dressed in dinner suits and others in shorts. Celebrity was very good but wouldn't bother with Royal Caribbean again (it was an older ship and showing it's age).
We went around the Caribbean on 3 and the Med on 1. Personally I didn't think the drinks packages were worth it, we wouldn't drink enough to justify it (for reference maybe a bottle of wine with dinner, couple of cocktails and maybe a couple of pints in the day), and I would think carefully on booking the ships excursions as they are expensive.