We are making our travel plans for sometime this May for about a 10-12 day trip. Has anyone done the suggested or modified Rick Steves' itinerary using only public transportation?
Your experience will be about 100% better with a car as these countries do not have the best public transport. You'll waste a lot of time. The roads are excellent as well.
You can certainly do the highlights by public transportation, but it takes more planning and you have less flexibility. There few trains in Slovenia and especially in Croatia on the coast. You could start in Ljubljana, day trip by bus or train to Lake Bled, train to Zagreb, bus to Plitvice Lakes, bus to Split, catamaran to Hvar and/or Korcula, and bus to Dubrovnik and fly home out of there. That itinerary is pretty straightforward, but twelve days for that is a bit tight.
I found driving in Slovenia and Croatia pretty easy, though. I did not even have a GPS, and I rarely got lost. In Croatia I had bought an expensive map ahead of time that I almost never looked at. In Slovenia I had only a free tourist map. Having a GPS would make getting around even easier, of course.
What you want to avoid is dropping a rental car off in a different country than you rented it in or you may wind up with steep drop-off charge. You take public transportation in Slovenia (or rent and return a car there) then take the train to Zagreb and rent a car there, then drop it in Dubrovnik. I've found car rental isn't too expensive in either country, thought perhaps it's much more expensive in the peak summer months.
While I can tell you that the buses from Ljubljana to Lake Bled are frequent and convenient, I've done the rest of my travel in this region by rental car or tour bus. The suggested modifications without a car still heavily lean toward at least part by car. I found the driving to be easy, no harder than the rest of Europe, and sometimes better.