. . . Some of the local car hire operators at AGP are a bit (or more than a bit!) rough. What they accept isn't necessarily what the police will accept if you get stopped . . .
No comment other than to say that experiences can indeed vary.
More important to characterize for the OP what the "typical" car rental experience out of AGP should be like. If one deals with the rental concessions located in the airport, the rental experience is just like that in the States. Service is prompt, organized, and documents are in order. It comes down to how you adapt to the local roads ( and the parking which is much tighter than many folks in the States are accustomed to ). The only trickery we've encountered is that some concessions tell you to fill the tank prior to return ( better deal to the renter ) while others tell you to bring the car in as empty as you can ( works better for the company ).
The one thing which is different in Spain is that the cars here tend to have more little scratches & dents which reflect the tighter parking and every day wear and tear put on the vehicles. The first couple of times I get a car with scratches/dinks which are not noted on the paperwork, I go back to the desk who checks the computer & tells me not to worry . . . it was a leap of faith for me, but we've not had problems in that regard either. Quite a contrast to car rental in Ireland where the vehicles as pristine & immaculate and they note every scratch before and after.
Barry