I have an American friend from Seattle who drove to Vancouver and flew to Cuba for vacation with his wife. The Cubans won't stamp a US passport so there's no proof you were there.
I had heard that too, and was therefore quite surprised and a little upset when they stamped mine. I had a very busy passport so I let it go thinking it would never be discovered, but right after 9-11 when I re-entered after a trip to Canada they carefully paged through my passport and discovered the stamp. Fortunately they didn't prosecute me for violating US Treasury regulations but I was quite concerned for a while. I have since replaced my passport.
I do know that others have not had their passports stamped while there. So it seems to depend on the whims of the immigration guy.
Back to the original posters question, I think Havana is a very very interesting place to visit. If you are a male traveling alone or with other males you will find the prostitutes to be unceasingly aggressive (for example, approaching you at your table while you're eating breakfast at a restaurant).
If you can avoid that (say if you're female or are traveling with one) it will be a fascinating place.
My biggest surprises when I was there (97) - Marlboro and Coca Cola everywhere. Not just for tourists. And the 'official' taxis were all late model Mercedes Benz' with leather interior. Really.
Speaking of taxis, every car on the road is an 'unofficial' taxi, in that if they see a foreigner walking everyone will offer a ride. We had a car filled with 8 people offer us a ride, had we accepted they would have left their passengers on the roadside, driven us wherever we wanted to go, and then returned for their passengers.
Most depressing moment - I was walking down the road and a man offered me his wife.
Bring plenty of cash if you're a US citizen because your credit cards and atm cards will not work there. (unless things have changed since 97). When I went I didn't bring enough money, fortunately one of the guys I was traveling with had a credit card from a non-US bank.
Despite all that, it is a very interesting place, and I think worth seeing soon beofre it changes completely.