These "new" Halo 3 maps were just released today, and just for the hell of it, I decided to get them. (I say "new" because they were scrapped from the original game.)
Foundry: 3.75/5 The worst map of the bunch and one of the worst in the game. Sure, it'd be fun for Forge users, but the default map is just lame. All it is is a warehouse full of boxes. You will find yourself getting shot in the back like crazy, and there are grenades flying everywhere. There's nowhere to go on this map. It's just deathmatch no matter what mode it is on Foundry.
Rat's Nest: 4.25/5 Sharing the name of the unpopular map from SOCOM 1 and 2, Rat's Nest is much like the campaign mode's Crow's Nest. Rat's nest is good for Capture the Flag, especially, and only decent for deathmatch games. Rat's Nest is a pretty big map. The outer edge of the map is all a track that you can use to transport around on vehicles. This is probably the most interesting part of the map. Especially when both teams have the flag and bump into eachother. It makes for an epic battle.
Standoff: 5/5 Standoff is easily the best new map, and probably the best map in Halo 3. Standoff is apparently a mix of Blood Gulch and Valhalla. To me, it has the landscape of Valhalla, and a bit of a High Ground look. Standoff is great on any mode, but it's absolutely perfect for Capture the Flag. Everything's great about this map.
Halo 3's Heroic Map Pack should probably only be bought by the more hardcore Halo 3 players. For ten dollars, you're getting two pretty average maps, and one fantastic one that's basically a mix of two maps you already have. Still, I think most people that have Halo 3 will buy these maps just for extra variety.