Premium gas becomes more important at low altitudes, in cold weather, or with modded cars. On a bone stock e46 at say 5000 feet altitude regular gas is probably the same.
On a cold day at sea level that premium gas is of much more value.
Any car with turbo or supercharger must use the highest octane gas available. If you have a CAI you might only see its effect with high octane gas.
Essentially the more Oxygen is in your engine the more important Octane becomes.