If your going for a felguard spec, I would highly recommend levelling with one build, then re-speccing at around 56 to get the Felguard. Just plan on using Voidy until then. The reason being, after you start using Felguard the point in Imp VW are wasted. 56 is the point where the Felguard really starts to shine due to abilities he gets, and the ability to pick up all the supporting talents.
If I were going to level a Demo 'lock I would probably use this until 56:
http://www.wowhead.com/?talent=IVMMZ0xIzIiz0es
Then switch to this:
http://www.wowhead.com/?talent=NVMMZbxhzIiz0es
Then follow this to 70:
http://www.wowhead.com/?talent=IqMMVMbZbgczIizMesz
Edit: Sorry, just realized I had messed up the first build. It's fixed now.
Edited, Aug 10th 2007 9:24am by Lathais