A quick run down description of each map...
Gemini- Most places call it the dud of the batch, but it doesn't look to bad. For those of you who just like to grab two guns and run, this is your map. It will be a decent Slayer map.
Relic- Going to be great for CTF and Assault. It's a pretty large map, but it's not too big. Reminds you of the Silent Cartographer from Halo:CE.
Backwash- Halo 2's Vietnam map. Lots of trees, fog, and similiar to the swamp single-player level in the first Halo. You can see and hear a 343 Guilty Spark clone flying around, and at one of the metal gates, you can hear the flood behind it. It's going to be a lot different from other maps due to the fog. Going to be awesome for Slayer games.
Terminal- Definitely a CTF/Assault map. Destructible trucks and cars ala Turf, the already infamous train in the middle...A decent sized map, due to different levels. Will likely feel similiar to Headlong.
Elongation- Longest from Halo: CE with a new coat of paint. Make sure you don't ride behind a crate the entire way, because they go down into holes like Colossus. Going to be a good map for CTF and Slayer.