All the Warhammer-community is following new space marines releases since it's the first codex in the 8th edition and there are a lot of new miniatures. We've started studying the new book (the review is out soon) and now - a couple of words about new units to make the waiting more interesting.
The main value of the codex is in the depth that it adds. These depth and variations were missing in the index. Now we again have chapter-tactics, relics, warlord traits, full-scale (but only one) psychic discipline and even our own strategems. All this stuff will be discussed in the review. However, you can see right now that all the chapter tactics are really usefull and it's hard to choose the best (at least, Salamanders are kind of boring). The most interesting part is strategemes - they really add depth into gameplay. THere are a lot of them (3 pages), and there are common for every chapter and some special - to represent chapter specifics better. You'll really want more command points to use them.
All new models are for the primaris marines. One of the most interesting units is Reivers. It's a kind of primaris-scouts. They can set up in 9 inches from the enemy, have bolters with -1 AP and 3 attacks in close combat if you take knives. But the main point is that they damage enemy's leadership by 1 while being close. So, you can use them in a very interesting way - getting closer to the enemy lines, demoralizing them and then unleashing firestorm from the heavy support units behind. Also, these guys can be effective themselves by pushing enemy units from the objectives.
Redemptor dreadnaught was shown even before the starter was released. This beast can choose from 2 major weapon options - machine gun with fearsome 12 shots (in fact, it's like 4 heavy bolters) and plasma cannon with Str8, which is spoiled by random D6 shots. It's interesting to point out that this model is not destroyed if over-charging fails, it only suffers a mortal wound. Machine-gun option looks more effective (gatlings, built into the fist is the same as a big one, but with 6 shots). However, the problem of this vehicle is not only in the high price but in the unchangeable power fist. If it could have a second range weapon, it would be far more effective. But now if you want to get into the close combat, you have to suffer a penalty to the shooting (all weapons are heavy). So, either way, you use this dreadnaught, half of its potential will be reduced.
Repulsor is the first and the only tank for primaris marines. It can carry 10 models and is equipped with weapons as hard as you could imagine. In general, you can choose from the same machine-guns as the Redemptor has and lascannons. Repulsor costs almost like a Land raider and is extremely durable. So, if you don't like Redemptor's model, you can easily substitute him with this tank. In addition to its firepower, it will take your warriors wherever you want. As a stationary anti-tank unit it acts like a Predator but costs far too more.
Agressors are something like Centurions. A heavy support unit effective only against light infantry. Both versions - machine guns with rocket launchers and flamers have an extremely big number of shots, but all with Str4 and without AP. So, they are very situative. However, they have power fists. So, if you take them closer to the enemy lines on the Repulsor, they can slash through the enemy lines. In fact, they are the best option to be transported in the Repulsor, since all other primaris units are effective from the long distance or can get to the enemy themselves.
It's expectable that in addition to the Primaris Captain and Librarian we'll also get an Apothecary and a Chaplain. The Chaplain seems to be one of the most charismatic GW models ever. They come their colleagues in stats, but with all the Primaris improvements.
Except for these ones, there are new weapons for the already existing units - 3 types of plasma guns for Hellblasters, special wargear for Primaris tactical squad, and plasma blasters for their jump version. So - we are waiting for these sets to appear in the pre-order in a week. This will finish up the Primaris marines releases. In general, you can use them as a separate army. But they are too expensive and don't have all the options, regular marines do. So, they don't substitute them but create a nice addition to the existing army.