We've always used Hertz - even for our European rentals. But when we did a comparison check with SixT (for a one-week, in-country rental in Croatia), the price difference (including no in-country drop-off charge by SixT) has us considering a rental with SixT this time. Of course, a web search (for "problems") turns up the bounty of dire stories (that would have us doing nothing on any front! ... flying, driving, buying anything). So I've come here to hear some sane and reasoned experiences -- both good and bad. Thanks!
My husband's maternal grandmother came from Croatia and we often visit family there. We have used SixT frequently with no problems at all. The staff is nice and we like getting luxury cars at a much cheaper price than we ever could in the USA. You can also get basic cheap cars too. They have a nice selection. Book directly through them for the best service. Croatia is a fantastic country to visit and the people are warm and welcoming.
A friend used them in the fall for a rental in Greece and was very pleased. We just used Gemüt.com for our Spain car rental and were very pleased with them also. They have a great information brochure about European car rentals on their website.
Sixt is a German company. They are my preferred option for car rentals. Used them lots and always had good service and nice cars.
Sixt has been among Rick's car rental recommendations for as long as I can remember.