Stephanie Rosenbloom recently had this to say in the NY Times:
Owned by the travel review behemoth TripAdvisor, Viator offers reasonably priced sightseeing tours and activities from a variety of people and companies almost everywhere on the map. Guides have been reviewed by travelers who have already taken the tour, and you may contact your guide before you book. Many will customize their tours to accommodate your interests.
Private tours include eight-hours in Bali with stops at Ubud; a spice and coffee plantation; Kintamani, an active volcano, and the Tanah Lot temple ($60), as well as a five-hour tour in Mexico City focused on the homes and work of Frida Kahlo and Diego Rivera, with stops at places such as Museo Casa Estudio Diego Rivera in San Ángel, the Anahuacalli building designed by Rivera and the Museo Dolores Olmedo in Xochimilco ($175 for up to four people). Professionally licensed guides have green tags, and if you like you can filter your searches to find them. I’ve used Viator for a couple of small group tours, but keep in mind that the company does not screen or conduct background checks on private guides. For more information: Tourguides.viator.com.