Hands down best transit I've experienced in the GTA. Clean, fast, and relatively cheap. Oh, and the trains are awesome! They're so quiet and have bathrooms and plenty of legroom. Service is usually very timely. If I could add anything, maybe there should be more of a weekend service? GO transit is kind of hard to get into unless you know the exact schedule and the system very well, it's designed for people to take as a routine thing, not for trips because you would have to plan carefully around the train schedule.
Also, they customer service is excellent, from helpful bus drivers trying to find connecting buses to ticket salespeople making sure I got my refund properly. I guess it shows they pay their workers well.