I don't think I could count the favorable discussions on one hand that I have seen on the internet about how useful Possessed Chaos Space Marines are. In game terms they are over-priced and under-perform.
Having bought a few of the CSM Battle Force Boxes I was 'blessed' with quite a few of these little buggers. I kept almost talking myself into putting together a unit but just never could bring myself to do it.
As I started fleshing out my units and started needing special weapons I knew I had discovered the perfect use for these ugly little blighters - conversions.
Need a powerfist? There are tons in a box of these guys. Lightning claw? Same deal. Power weapon? Got that too. Scratchbuilding a sorcerer or unit champion or even a lord - this box is full or awesome heads, torsos, legs, and arms. Also backpacks. Price of Raptors got you down? Each box of these guys comes with two winged backpacks - instant Raptor.
The list goes on and on for what you can do with all the bitz you get out of a few units of these guys. Here is a bit of inspiration for you - my Khorne Berzker Skull Champion.
I used the standard Khorne Berzerker legs, chest, shoulder pads, and bolter arm. For the head and arm I grabbed a few furious looking bitz from my pile of Possessed bitz. Bam - instant champion.