I will be traveling with my wife and 15 year old son for the month of June. We have traveled Europe before but only ever by train. As I have been reading, a car might be a good alternative as we are planning covering a good deal of both countries. We wanted to start in Bled and end up in Dubrovnik. Is driving this worthwhile or would the frustration level cause me to go insane?
A car gives you much more flexibility, especially in Croatia. I've driven in both countries and roads are generally very good. A GPS is always helpful. If the fee to pick up in Bled and drop off in Dubrovnik is too high, then take bus from Bled to Ljubljana, train to Zagreb or Rijeka and pick up a car there. If you rent from Uniline, a Croatian rental car company, they'll pick you up at the train station during business hours (must request in advance) and they charge no drop off fee for rentals of 3 days or more within Croatia.
I second everything Frank said (although I have no direct experience with Uniline). A lot of the main highways are pretty new, having been rebuilt after the war in the 1990s. I can't, however, speak to how traffic might be in June, as we were there in March. But all things considered, Slovenia/Croatia is one part of the world where having a car makes sense, as the public transportation options can be limiting. Have a great trip!