I took my car 2001 Audi A6 for service and my mechanic changed the coolant with a regular coolant. Should I be worried? I have heard my people saying never put any other coolant other than G12 and then there are those who says it does not matter. Is there any truth in that?

 

Yes, it matters very much.
G12 and other "extended life" coolants are made specifically to work in an aluminium engine block and to not harm the radiator, seals, pump bearings, etc. Worst case is you mix the two types - it forms a gell-like sludge that will block up coolant ports and really screw things up. You shouldn't even use tap water to mix with the right coolant, only distilled water, as the minerals in tap water can precipitate out and cause damage.
Get it completely flushed out and put the right stuff in. And seriously consider finding a mechanic who knows enough about Audis to not make that kind of mistake in the first place...
Jim