I started my Chaos Marine army when GW changed their paints range and modelled up some Fallen using some Dark Angels Veterans pieces kit bashed with a box of basic Chaos Marines, just so I could compare the new paint range to the previous paints that I had been using and also, for some modelling fun.
Although this was never intended as the start of an army, it has developed that way and I have now got 15 Chaos Marines, 4 bikers, a Terminator Lord and a Sorcerer painted up as Fallen Dark Angels.
To give myself a break from painting Green Power Armour all the time (2 Dark Angels armies?...) I have turned my Chaos army into a Black Crusade of different Chapters/Gods and now also have 12 Khorne Berzerkers, 5 Raptors (in World Eater colours), 10 Cultists and have also started on some Slaaneshi Marines of my own Chapter in my Crusade.
I have noticed that it is lacking in the Heavy Support department, I did buy some Obliterators in the summer, but found that they are a pain to try to transport around as I found the weapons on their arms keep falling off, so I don't bother with them. I do love the Defiler model, so may go for that or a Forgefiend to deal with some HS.
I've only played a couple of games with this army, my bikers get Mark of Khorne, my DA Marines, have used Mark of Nurgle, though I shall be trying all Marks before I settle on one for them.