Driving from Italy to Istria

I've not driven this particular section of Italy/Slovenia/Croatia, but I did drive from Venice to Ljubljana. Not sure where you heard about the bad news regarding the route. Slovenia is much cleaner and nicer than it's two neighbors and I can't imagine that the route you're speaking of is any worse. In general, roads in Europe are way better than they are here in the states. Perhaps try to see if Google Maps allows you to view Street View on that particular route. I've driven (not personally) through many parts of Slovenia and have never encountered problems with the roads before.

I picked up and returned at car at Portoroz, Slovenia, for driving a good loop through that country, and had no driving issues. I would not make any negative comparison to driving in other parts of Europe. I recall the main A1 highway (which you probably won't take) being impressively new and pleasant. The worst traffic I've encountered in Europe has been along Croatia's coastal route in high season and around the Amalfi Coast of Italy

I have driven that route, except I took a detour to stop in Piran (which is a worthwhile stop).

I can't think of any reason why anyone would lead you to expect anything unusual about driving there. The roads in Slovenia are perfectly fine.

The only thing to consider is the need for a vignette to drive on the toll roads in Slovenia. Your route does include the H5, which is a toll road. You should be able to buy a vignette at the border. I believe you have to buy a weekly vignette for 7.5 euros; I don't know if they have shorter-term options.

Thanks everyone! I will follow your advice over a guidebook. I have been having some trouble with rental car companies not wanting me to drive in slovenia and some who said they only allow certain (cheap) cars to go to Slovenia. Do you remember which company you used? Thanks for the suggestion of google maps! I did "walk" a section of the route and it looks totally normal. Any idea why google maps puts an ! in a triangle and says "warning this route goes through Slovenia"?
Thanks again for your thoughts!

Amy, my rental car was arranged by a travel company I worked with, so I don't know what company it was. I rented in Zagreb, and there was no restriction about driving in any other country. (I went to Bosnia as well.)

I believe Google maps always includes alerts about border crossings when it gives directions.

Good luck!