Never have I switched out of Fliction, 10 to 70.
NEVER have I been side-by-side with a Demo lock and had him Kill faster, put out more Total Damage in a 5man/raid, grind faster or more efficiently. Demo's Pet Quickcast does make for a safer build in most situations if you're quick enough, and the term FelNub, although a relatively degrading term, refers to a Warlock who just sends in his Pet and maybe throws on a DoT or two... this tactic DOES work, but it's even less effective than a good Demo Lock.
The main problem I have with Demo lock for grind, especially at first, is that your pet WILL run out of mana LONG before you do, and even with techs to get his mana back, you're looking at a constant merry-go-round of Improved LT + ManaFeed then Life Drain just to keep him rolling at a decent pace. As you reach upwards of 65 this seems to be less and less a problem, but especially in more hairy situations (2 monsters?! OH NOEZ) you'll see a Felly drain his mana seriously quickly.
In BG, Demo fails the Damage/Kill test, as well as large WPVP encounters. Where Demo (and Destro for that matter) seem to shine is in 2v2/3v3 Arena, small WPVP confrontations (duels,1v1/2v2), and Farming.
This being said, the MAIN thing I have against Demo is that it IS too Pet dependant. 90% of Demo Locks will agree, and of that group, 90% will say they are fine with that! Demo is a very unique and specific tree (as is Fliction) which fits certain play-styles.
It's really all about what you want to do.
Having been in your exact situation, however (you can probobly find the exact same question posted by me on these boards somewhere), I can tell you that you'll have absolutely 100% no problem leveling efficiently, bringing damage to groups, and holding your own in PVP on the trip up to 70 (and beyond.) There is also a certain level range (about 3-5 levels below the Lock) for which Affliction lock is the king of Farming (DoT DoT DoT DoT, Change target, Repeat x5, IHoT, get new Targets -- can do about 15-20 this way before break). For lower monsters, however, as stated the fact that a Demo can have his pet kill for free makes it a good choice.
One thing I WILL warn you about if you stick to your Fliction background, however, is your first few levels in BC. Hellfire Peninsula is a ***** of a place to level as a Fliction/Destro warlock. It just is. The good news here is that you will still be bringing great utility/damage to Hellfire instances, and you will be out of this level range (61 or 62 you can move out) insanely quickly if you just do the chain quests.