Assembly: 4/5 Assembly is virtually a Midship remake from Halo 2, with a bit of a Gemini feel, too. Although Bungie said nothing about Assembly supposedly being a remake, it most definitely is. While it's not a bad thing, I was caught off guard and wasn't too pleased at first. Assembly is a decent map, but expect to get shot in the back a lot on this map, seeing as how it's a round-like map.
Orbital: 5/5 Quite possibly my favorite Halo 3 map. Orbital is a great asymmetrical map that has a Star Wars feel to it. This map is great for Team Slayer and Capture the Flag. Not much else can be said but that it is quite epic. It also features a rocket launcher, sniper, and tricky crouch-jump area.
Sand Box: 4/5 In itself, Sand Box isn't fun or impressive. Playing this map in a playlist with Capture the Flag is completely trashy. When the opposing team is driving around the map on a Warthog while someone else is spawn camping with a Brute Chopper, it makes it virtually impossible to escape. However, Sand Box as a Forge map is better than Foundry for the simple fact that it has three levels. A monster air level, a bigger-than-Foundry under level, and a decent sized ground level. My main gripe about this map in Forge is that it the pieces are kind of ugly for the most part. Foundry's pieces are just more appealing.
The Mythic Map Pack, to me, isn't a very strong pack. Forging around on Sand Box is fun, but you can't really build a huge map on the "air" part of the map because of a lack of pieces. The biggest map I've made on Sand Box is a Vacant remake from Call of Duty 4. That barely worked. I recommend downloading this map pack if you're a huge Forge user or Halo 3 fan. Otherwise, you should probably wait until the price goes down.