Who has taken the ferry from Venice to, say, Rovanj? I'm thinking of visiting Istria for a few days next May. Is it easier to drive? Thanks.

I have not, but here is the ferry operator for Venice to Rovinj:
http://www.venezialines.com/
Driving would depend on your itinerary and rental contract for crossing borders.

The ride from Venice to the border reminds me of the landscape at the end of "American Graffiti": flat and industrialized agriculture. Istria looks like Chianti or Napa and is well worth having a car for. I love ferries, but you do miss the landscape.

If visiting for a few days, then a car is your best friend. In two days in May, you can cover most of Istria. In three days, you can include some of Kvarner/ Go for four days, and you can visit Plitvice too.
In May, roads are not too busy, making driving so so easy.

What is the weather like in JIune or September? Would you recommend driving throughout Croatia or train or bus?

We were in Croatia several years ago and thought it was wonderful. We spent most of the trip in Istria.We had a car and traveled to Trogir. we didn't like it much so we drove back to Rovinj. We stayed in Porec and Rovinj in Istria. Istria had been part of Italy so the signs are bi lingual and the food is great and much of it is Italian.
I believe that a car is the best way to travel.

wenday, you should start as new thread. June and September are both nice, though early June and late September could mean cool sea temps. Sept should mean less crowds.
Transportation depends on your itinerary, but the bus and ferry system are both excellent, and the train not very extensive.