We've been to two. The first at the Lanzarote Princess was quite a formal affair with a sit down multi-course dinner all waiter served (everyone claps as each course is brought out) and very drawn out. I remember we had two very young children then and as anyone with kids knows, there is a finite time in which they'll sit still at a dinner table, so we ended up leaving about half way (2 hours) through.
The second at the Playa La Arena was kind of a posh buffet (fillet steak, smoked salmon, king prawns) and the Friexenet Cordon Negro, wine, beer and soft drinks were complimentary all the time you were at the table. This suited us much better and was very enjoyable.
Worth checking it's all included (though it usually is with a package booking) as they can cost £50+ per head when you DIY it, and they're usually compulsory.