Over a year ago, I hired Swan Roofing to replace the shingles on my roof. To this day, I have not found any leaks. He and the two young men working for him seem to have done everything properly. They replaced the shingles, caulked certain areas of the roof, installed a metal valley, cleaned up well around the house, and hauled the old shingles away. On top of that, their work is guaranteed for ten years. I don't really have any major complaints to make.
Perhaps they could have done a "straighter" job in some places where they put the shingles on. I have noticed that other roofers who did other similar houses on my street seemed to do a "cleaner" and "straighter" job on the shingles along the edges of the roof. Also, the shingles near the metal valley could have been "tacked down" tighter so that they are closer to the metal valley.
Nevertheless, my roof has not leaked and it is over a year since the installation was done. I assume this is what counts the most.