My wife and I have used Viator on many occasions and have never, to date, been disappointed. The prices are more expensive than you might reasonably pay locally but for that you buy peace of mind and quality assurance. The trips are easy to book on-line and the documentation is for the most part clear (although information regarding pick up times and details is usually left to the local operator and arrangements can and do vary). Once arranged pick up is invariably on-time. Our Guides have been uniformly excellent. Knowledgable and pleasant without being intrusive. All have had excellent English.