Montenegro

We did this and it was fine. You need to remember that in 1991 or thereabouts Montenegro and Croatia were at war as part of the Croatian war of independence. There are still some border disputes we were told but the countries have diplomatic relations. We were told we may be scrutinized at the border because our car had Croatian plates. I think everyone gets scrutinized as the line to cross was about 2 hours. While driving in Montenegro we got nabbed in a speed trap. I don't think having Croatian plates was a factor. But, watch the changing speed limits. We loved our time in Montenegro. Our most memorable day was spent doing this: http://www.montenegro4sail.com.

Be sure to notify the car rental company that you want to do this, preferably at the time you make your reservation so that you don't get hit with any unexpected fees. Also, note that you are likely to be driving throigh Bosnia-Hercegovina as well; it has a little bit of the Adriatic coast.

I really enjoyed Montenegro but thought that the historic districts if Budva and Kotoo were pretty similar in appearance. If I had time for two cities in Montenegro, I'd substitue another one for Budva just for the sake of variety, though the drive between the two is very scenic.

When we crossed the border about a year ago it took 10 minutes.