Traveling to Spain for Honeymoon in June - planning on going to the bigger cities - Madrid, Barcelona, Seville, Granada, and couple others. How easy is it to rent a car and travel that way? Or is it just best to use public transportation - trains, etc.

You would be best to take the great rains between the larger cities and perhaps fly in and out of Madrid/ Barcelona and do a circle trip.

From one of the southern cities, ( Sevilla/Malaga) you could rent a car a few days if you wanted to have more freedom to visit the special small towns/ beaches where I feel a car is totally necessary and an advantage.

You do not want a car IN Madrid, Barcelona or Sevilla. Driving there is impossible expensive, and really does not help you tour and see things. Parking is very expensive. Granada and Málaga are quite civilized as there are parking garages when you do the one/two day stopover on your way elsewhere.
Another option is to end the circle in Granada and fly to Barcelona from there..

If this is next month this year, you may want to get moving on the hotel reservations. Check out the parador system.. many are converted castles. To get to many of these you will need a car. However, the ones near Madrid in the hub around the city will be easy to get to on public transportation.

www.paradores.es

casanova is right that in cities, a car would be not only useless but a hindrance. Between cities, high speed trains are a good option.

However, all over Spain you find a very scenic countryside and the Spanish highways are among the best in Europe (scenic, hardly congested). In fact, the Paradores system was created specially for roadtrips. So, if you do not rent a car (convertible?) and drive for a few days, you would miss something.

From your itinerary, I understand you will stay for at least three weeks in Spain, so take a few days to explore the countryside. It is especially beautiful around Madrid, in Andalucia and north of Barcelona, at the Costa Brava.

Are you from the U.S.? Is driving the essential part of the trip experience or is it just one of the several means of transportation? I am in the latter group and I am with lincasanova by not using a car in big cities. However, I realized that many travelers from the U.S. are in the former group.

What you would notice is 1) the remoteness of Barcelona with respect to the other destinations and 2) that Granada is a cul-de-sac type of location when approached by land. One way to address these issues is to rationalize the itinerary by flying into Barcelona, fly to Granada, then do a groud trip by driving in Andalucia, drop the car in Sevilla and continue onto Madrid without car. You can also do this in opposite direction.

What is best depends on what you want to get out of the trip and the value you place on different factors.