No specific bad experience, just general impression. And yes, people are nice (like everywhere else I have been).
It is just my opinion about value for money. This is my experience.
On average, meal will cost you about €10. Some of my friends are vegetarian. If you are, you'll be struggling...at the same price.

There are big variations in quality between different places, but you'll still be expected to pay the same price.
Casa particulares, room will cost you from €20 in south (Playa Giron), €25 in Vinales area, up to €30 in Havana. It is very basic. Don't expect toilet seat, toilet paper, soap or enough water pressure in the shower. There are dodgy electrical instalations in showers...but considering it is 110V - you will live to tell the story.

And don't worry, most of that money won't go to those people. I was told that government takes 2/3 of what you pay to landlords. And if you happen to be in group of four or more...you cannot stay all in the same place. They'll split you in twos and send to different casas.
There are variations in quality between casas, but you'll still pay the same price.
Taxi in Havana will cost you about €1 per km.
T-shirt in Maria la Gorda will cost you €18. In Havana about €7.
Postcard is €0.50 and stamp for Europe is €0.75.
Scubies will have to pay €50 per dive. You cannot negotiate rate for freediver as (so I was told) "You cannot dive for free in Cuba" (Cayo Levisa, diving instructor).
Enterance to some beaches (Cayo Jutias) will cost you €5...and that is only enterance. Everything else you have to pay additionally.
Diving spots around Playa Giron are free (and you can do some deep diving as well). For some you have to pay €1, but you get toilet facilities, shower, shade and chairs.
Beaches east of Havana are free, but maybe not best for diving. If in Havana, you can try Marina Hemingway. About 200m out, you'll find reef drops down to 15m. It is very difficult to find weights for rental anywhere in Cuba. Your best bet would be to talk to local spearos. North coast water temperature in December was about 26C. South coast, surface water temperature was about 27C and few meters down 28C (temperature inversion, very pleasant phenomena

).
I am not sure about spearing licence. I guess officially you would need one. Spearfishing seems to be one of the few things that government does not fully control.
Any card transaction will cost you 12% of total sum. If it is american card or you are exchanging $, it'll cost you 20%. Take €, cash...and plenty of it. You'll have to exchange your currency to convertible pesos. It works out that 1 CUC=€1. Prices are often marked in $ (even on card transactions), but now it means €.
Locals have different currency that you cannot officially use. I think the biggest inconvenience for me was the fact that you cannot buy things (especially food) just anywhere you want. There are shops for tourists and shops for locals. And as you go outside Havana, you find less options. If your Spanish is not too bad, you could get away with it. Otherwise, you might have to catch a lobster.
Smallest car, rental price per day (if more than 7 days, no miles limit) is €75.
Petrol price is about €0.80-€0.90 (cannot quite remember).
Traffic penalty for going through the (possibly

) only pedestrian traffic light in Cuba (with no pedestrians on crossing), attached to some overhead lines and very difficult to spot if you do not know it is there, doing 20km/h...€30.

If you are driving, beware - no traffic rules for locals.
Cuba is poor country with communist system and under sanctions. You will probably sympathise with people, but don't expect good quality products/services. Government fully controls pretty much everything. There is no private initiative. If you do have chance of negotiating the price - it is probably illegal. Prices are set to whatever government thinks they should be. If something is good, you'll very probably have to queue for it.
Waiting time at entry passport control was about 3 hrs for people from our plane (747). Exit of country tax €25.
In short, you get former Soviet Union products/services at London prices.
Complaints, refunds...in general...don't bother.
You can buy Cuban rum in Sainsbury's and cigars at Madrid airport (maybe somewhere in London as well?).
And again...yes, people are very nice.
