That map needs to be re-thought. The fundamental problem with Narrows is that if one team takes initiative and stays solid on the high bridge, it's hard for the other to come back. But the team always has a chance because there's a relatively accessible low bridge as well as a lift for escape and regrouping. From what I can see, this custom Sandbox map does not. Given Bungie's previous record of starting players without decent weapons, I don't see the map becoming popular except in hardcore setups like the Snipers, MLG, or SWAT playlists. Just an educated guess.
Anyway, I'm certainly looking forward to the ability to recreate lots of maps from Halo and Halo 2 without the need to take up Bungie's time or more of my money. Good update.
[Edited on 03.27.2009 8:57 PM PDT]