I am currently enrolled in the summer Masters program at SLU in Madrid. Last summer was my first summer and it went pretty well. The courses are interesting and varied and the great majority of professors are native. Most of them are professors at universities in Madrid who teach at SLU during the summer break.I really enjoyed all three of the professors I had last summer. My only major complaint is that there is a required class on teaching methods. I don't see how it really fits into a program in Spanish language and literature, even though most of the students in the summer are teachers. I figure that MAT programs exist for a reason and I would be in one of them if I wanted to take more education related courses.
Overall, it is a nice program and they are now even expanding it into the academic year, so you could take grad courses all year round if you wanted. If you have any more specific questions, private message me or email me and I will be happy to answer them.