The key for you is to look at the testing facility. Do some research to make sure there are no issues, but there is no reason to think you will have a poor experience.

Jim is quite right...the test should be the same, check out the facility (or facilities). You may have more than one option and you could choose the best one...

Are the monitors 12 inch things from the 1990s?

Is the location loud?

Are the staff familiar with the identification you will be presenting (since it will not be what they usually see?)

You can go a long way with a little prevention. Remember, it is all on you make sure that everything is acceptable. There are standards that these places should meet but you are the one going to business school so you have the most at stake.

With that said, I would think that conditions in Spain would not be pretty good at their testing centers...does anyone who has tested there before have anything to say about the conditions?

Of course the best planning does not help when you are taking the test in Munich (as my tutoring student was) and they discover an unexploded bomb from WWII almost directly under the test center...crazy things may happen but take care of what you can control.

Thanks for the tips!
It would indeed be very usefull if somebody who has done the GMAT in spain could tell me about it?