Tim pointed a website to me about "
Travel like a local", where you can ask locals for some advise about your next travel destination.
So, are local people really the best choice for you? I would say: not in general. It depends on your needs.
The average traveler comes to a place the first time and don't want to miss the highlights. But, have you ever asked someone from Paris about the opening hours of the Eiffel Tower? Guess how many citizens of Saigon have been to one of the museums recently? When I lived in Phnom Penh, I never made it to the National Museum (but I visited it years before when I came as a tourist). So locals are not a help for the first visit and the basic sightseeing. Who else then? Your professional tour operator. They know what tourists like (and don't like), they have the best rates and information. It's their business.
Ok, you have been in Saigon once and now you come back and want to get a deeper look into the citys life. Are locals your best guides now? Maybe. Again, it depends what you expect. You want to have a food tasting in Chinatown? Locals can take you to a restaurant they know, but is it the one you really want? Maybe, if you like really authentic food (with all the risk, but fun too).
Locals live here, and daily life isn't for them (or us, as I am a kind of local too) as exciting as it might be for a tourist. Like driving on a moto bike: It's common for us, but maybe a joyride for a tourist. On the other hand, there is nothing more boring for me as a cyclo ride (and usually f** hot too).
If you are a backpacker and want to save money and get a real authentic experience, then should should ask locals for sure. They know the cheapest places (even cheaper then mentioned in the Lonely Planet), and the best as well.