I don't like current Codex Chaos Space Marines, for many-many reasons. One of them - there are too many places where I can't see any logic. Because of this I bought older Codex, written by Andy Chambers, Graham McNeill and others. And that Codex is totally awesome! It's a bit old to play with it though, but there are many things I like. Marks of the Dark Gods for example.
Current Codex, 4th edition
In current Codex, released in 2007, Marks can be used everywhere and by everyone. And as for me it's breaking fluff by all means possible. For example, you can summon pack of Daemonettes without scatter on Mark of Khorne, or put your Sorcerer with Mark of Tzeench with 9 Berzerkers into Land Raider. How powerful and cruel Chaos Lord can allow this to happen? How is it possible when Dark Gods are watching you? I don't know, but it seems to me that Codex' author wanted to play Orks too much, so he made ones, using Chaos kits. But let's return to how Mark works. Summoned Daemons and Terminators can arrive on Mark without scattering, and also Mark somehow is visible from the transport vehicles, and adds some abilities to their bearers. That's it. No more restrictions, no more options, no more advantages.
Codex Chaos Space Marines 2002
In this older Codex everything looks logical and close to perfect, almost like it really would be in Eye of Terror. Do you know that Khorne and Slaanesh are mortal enemies, like Tzeench and Nurgle? So, if your army's leader have Mark of one of four Gods - your army cannot contain any units, dedicated to God's mortal enemy. Of course it can't, just because your leader will kill all those bastards in the name of His favored God!
Next feature, Daemons can be summoned only onto Mark of their God or onto Chaos Undivided Mark. In Codex 2007 I always wondered why my Daemonettes would not attack unit of Berzerkers with Icon that summoned them, or why should they come at all? In Codex 2002 everything is just fine, like it should really be.
And my favorite, Daemon Icon. It's powerful Daemon binding artifact, and single Daemons unit can be nominated as target for this Mark. Then, when time comes, you can free those Daemons without rolling reserves or scatter, they're just coming out at the start of your turn. I like both how it works on the battlefield and how it sounds from the fluff point of view. But where did Chaos Marines lost it in 5 years from 2002 to 2007?...
Codex 2002 is a bit outdated, and as for me can be fairly used only in combination with Rulebook 4 and older enemy Codexes. It's navigation is not that simple, it's unit costs does not make any sense against Codex Space Marines 2008 or Space Wolves 2009, so heavy refresh is required before making it playable. But it's so awesome when comparing against current Codex Chaos Space Marines, with so many interesting features, and real feeling that it's Chaos Legions, cruel and glorious warriors of the Dark Gods, while current Codex 2007 makes it feel like you're playing some Chaos Orks or something. So I will better wait for Codex Blood Angels and will use it for my Night Lords army until next Chaos Codex will be released. It will represent Night Lords Legion a way better than Codex Chaos Space