For anyone that didn't know you can now buy the Mythic map pack from Xbox Live.

They are ok IMO. There are 3 new maps:

Sandbox:
Open profile map, good for capture the flag etc.

Orbital:
Kinda like a double version of Narrows. Not bad but I've only played it twice so far.

Assembly:
Small and circular with several levels. IMO the worst as its too small for the amount they have put into it (you'll understand when you see it!)