Less than happy is a bit of an understatement--it's a flat-out violation of the rental agreement. And it's not just New EU states--Hertz and Avis in Germany and Switzerland, for example, often won't allow cars (particularly expensive ones) to be taken to Italy or Spain.

The costs of one-way rentals vary widely according to the countries involved, and there are some interesting wrinkles: at Geneva, for example, it's possible to rent a car in either the French or the Swiss part of the airport, with massive impact on the cost of returning the car to, say, Paris.