When spark plugs begin to fail, they can't properly ignite the fuel in your cylinders properly. Due to this, it can cause a multitude of symptoms.  
Rough Idle
Poor Fuel Economy
Poor Acceleration
Smell of Fuel from Exhaust 
Hard to Start 
Engine Misfire 
 
There's a possibility that it's something more serious than just your plugs. Checking your plugs is a good first step to determine if it's something worse that needs addressing immediately. 
 
BMW has a regular service interval of 60,000 miles for the OEM spark plugs. They can fail well before then, or even later, so it's worth checking.  Also, inspection of your spark plugs lends insight into the health of your engine.