We have rented them with the car. Sometimes it's a bit of a hassle at pick up to actually get a seat that works (there's a lot of demand for them, especially during holiday season, i.e. July/August). The person who hands the car over often has no clue about these seats, so be prepared to check and install yourself. We have been given ones with missing buckles and straps (both in the US and in Europe), so make sure you check and get the right stuff right away.
In Europes all car seats need to be certified by the EU for savety. So when we moved to Austria we got rid of our US car seat (then we had only one child) and got one here that was legal here, too. I don't know if it's lawful to travel with US car-seats on a holiday here, but i would assume it's fine.
The colour and design of carseats on average is much brighter and joyful in Europe than in the US, i find. I like Europe's better, but then, i am European, so biased...
Have great travels!
D.