I wouldn't use Milkshape to model levels/environments. You can use 3dsMax, but it's not perfect either. Level/world editors really ought to enable you to sketch out the basic shape of the level quick. Maplet isn't advanced enough yet, but it's the right TYPE of tool.
I would be using Halo's Cartography Shop if it didn't crash a lot for me. If it doesn't crash for you, I highly recommend it. Or you might want to look into creating your own level editor.
I would be using Halo's Cartography Shop if it didn't crash a lot for me. If it doesn't crash for you, I highly recommend it. Or you might want to look into creating your own level editor.