Ok
Ive had a bit of experience with this myself and after repeated trips to the dealer, heres what finally apears to have done the trick on mine. They replaced the upper intake manifold. Seems there was a minute vacuum leak that took a fair bit of testing to find.
If you are getting a CEL there is something amiss in your system and resetting it yourself by disconnecting the battery is foolish (IMHO) because if there is a serious problem, it is not being addressed and you run the risk of warranty void.
I believe that the only way to protect ourselves as owners is through documentation of every problem that occurs.
Besides, Its under warranty for 50,000 mi and Mazda gives you a loaner each time so give them the chance to make a terrific car perfect. If they fail you'll have all the ammo you need to make them take it back! :D
Its a bit aggrevating I must admit but, perserverance is a virtue for this great car!
BTW, I now seem to be getting 20 mpg combined!!! :D :D :D
Regards!

 

Had my 8 to the dealer yesterday. They say the CEL was because the car thought my thermostat was stuck open. They cleared it and all appears well for now.