I have used ALPI in STI. I'll pm you. Tom
I can speak for this school too. I spent two months there about 8 years ago, and I enjoyed it so much that I recommended it two a couple of friends. Both of whom did the program at ALPI and had very positive experiences as well.
I also recommend doing a class in either Santiago or Santo Domingo, as opposed to one of the tourist areas. The idea behind immersion courses is to take you out of your comfort zone and put you in a position where you are forced to speak the local language. Therefore, doing an immersion course in a tourist areas kind of defeats the purpose IMO.