Is Yacht Club worth the extra money? I would have to say YES.
It cost us about $1000 per person above the cost of a normal balcony suite. We arrived early for embarkation and were met by a butler who took us straight up to our room. It was a bit larger than a normal balcony suite with plenty or storage room. Very well appointed and the bathroom had a bath which was nice. We chose a cabin on the port side as we got better views when we were passing land from that side.
Yacht Club entitled us to the free mini bar, room service, daily newspaper, priority embarkation at ports, use of the Top Sail bar, our own pool, bar and spa on the top deck and dining in the L'Olivio restaurant. The service in all of these was impeccable and the food in L'Olivio was outstanding. I saw some people paying for premium wines but there was nothing wrong with the free wines they served.
Without exception all the Yacht Club staff were amazing, polite and went out of their way to help with any problem. I can't speak highly enough of them. I don't know what the service was like in the rest of the ship so can't comment on that.
The ship is huge but didn't seem too crowded. It was spotless and there were always staff cleaning inside and outside every day. The passengers were mostly European so we didn't get chatting to many of them. Also with people embarking and disembarking at every port there were always new faces on board. It's not the type of cruise where you can get too friendly with people. Be prepared for no lift etiquette - they push onto every lift without allowing people to get out first.
All the shows except one were fantastic (one was opera which wasn't our taste). We didn't really frequent the other bars on the ship as we would have had to pay for alcohol there, whereas in the Top Sail bar most of it was free. They had a pretty good selection of wines, cocktails, beers etc all free. The Top Sail bar has a great view as it is right at the top and front of the ship. The only downside was that it was the only bar where we could drink for free and you can't take your drinks out of there. There's also no entertainment to speak of.
We did try to go to the gym a couple of times but all the walking machines and bikes were being used. It was disappointing they didn't have more of them.
We did shore excursions in Naples (Pompeii which was great) and Tunisia (shopping tour which was okay). At the other ports we just shopped and did our own thing. The shore excursions aren't cheap (ranging from 40 to 80 Euro) and I'm sure if you researched you could organise them yourself and save some money. We just did the ship's tours as they were convenient. A small criticism was that there wasn't much information provided about the ports, and no port talks beforehand.
In short, if you want a relatively quiet, peaceful time, aren't a party animal and are prepared to pay extra for luxury, good food and privileges then the Yacht Club is definitely worth it.