It's a fascinating place. We did a mixture of resort in Varadero and a bit on our own - Havana, Bay of Pigs etc.
Pros
The beaches and sea are absolutely beautiful. One of the nicest places we have ever swam in the world. Excellent snorkelling in the Bay of Pigs. The music is just fantastic. They do a damn good mojito and rum is dirt cheap.
Cons
The food is pretty awful. We are quite easily pleased food wise and don't have any dietary requirements or foods we don't like but we found we were happy to get back home to eat and that never happens when we go on holiday. We stayed in a 5* resort in Varadero and still the food was not great. It was edible and we didn't get sick or anything but we just didn't really enjoy it. The quality is not good and the variety quite limited. We did go to a couple of local restaurants where the food was better so worth doing that.
The worst thing for me is how poor the place is. You see people queuing for their rations. Some queue for ages. The shops are often bare by lunchtime. We got chatting to one of the entertainment staff in our hotel and she was talking about their lives and how poor they are and she was so grateful that we left her our shampoo and shower gel and toothpaste when we left as they just can't get them. We also left a lilo for her to give to her son to play on and she cried because she was so grateful. It made me really sad.
Tips
If you do decide to go, do try and see a bit out of the resorts as it is worth it. Also take GBP or USD and have plenty of cash. Many places don't take card still and the ATMs are incredibly temperamental. You will always get a better rate using foreign currency than pesos as locals can use foreign currency in certain shops and get more items. They've done away with the two currency system now so it's just pesos or foreign but foreign is better.
Personally I would be tempted to visit Havana again but would not go for another holiday there. I'm happy that I've been but would not be tempted to go back and I rarely feel like that about places I've been.
Sorry that sounds a bit negative but trying to give an honest opinion. Hope it helps!