I come from a long line of pork producers, so here's our favorite ways to cook pork loin.
My mom (world's best cook) uses this method: Marinade it overnight in an oven bag made for roasting turkeys. (she uses a mixture of season salt, garlic, onion powder, soy sauce and sesame oil.) Then braise it on the grill to seal in the juices. Next she returns it to the oven bag and roasts it in either the oven or a large roaster(depending on the number of loins she's preparing). I'm not sure how low she sets the roaster, but I'm thinking 300 to 325. It usually takes a couple of hours. The great part about this method is that none of the juices evaporate, the remain in the bag and the meat cooks in them. The result is so tender and juicy. This is the family favorite for our Christmas in July party at my parents' farm. Last summer we had 130 family attending. What a great time!
Here at home, we tend to cut the loins into very thick loin chops (1 1/2 inches thick) and marinade them in a McCormick Grillmates marinade packet - Our favorite is Brown Sugar Bourbon. My husband grills them on the gas grill until they are just done. Remember the USDA lowered the safe cooking temp for pork to 145 degrees F. Remember to let it set for 5 minutes before you cut any meat, so the juices can be reabsorbed into the meat fibers.