Both Demo and Affliction are great levelling specs. It seems more people prefer Affliction, especially before Outlands, but both work well if played right. Heck, even Destro works, you just drink alot.
If you plan on levelling Demo, you will need to respec at Felguard time. Having the Imp VW is a huge help to a levelling Demo Lock. You may want to grap Imp Corr and Imp Drain Soul First though. Imp Drain Soul will help your VW keep aggro. You shouldn't really need the Imp Health Funnel until you get your Felguard, so you could drop those an a point from Imp HS to pick up your Imp VW while levelling.
Once you get to a Felguard level, your Demo Tree looks pretty good, but is missing at least 1/3 in Mana Feed. If you don't take this, your Felguard will always be OOM. You really should have Imp SB and Bane if you plan on running Instances and things. They are really great damage boosters. Since you have felguard or VW you shouldn't be getting hit that often, so Fel Concentration is a waste. It's more of a Drain Tankers talent.
If it was me, I'd level like this:
http://www.worldofwarcraft.com/info/classes/warlock/talents.html?0502000000000000000000050330133250100501350000000000000000000000
Then at that point re-spec to Felguard spec.
Edited, Mar 21st 2008 9:52am by Lathais