I've been messing around with all 3 specs in the past couple days, and Destruction is now my favorite spec for dungeons and questing. On raid bosses Demonology can pull ahead, and it suffers less from downtime issues when you have to move since your pets keep chomping away.
I just can't warm up to Affliction. You build the dots and channel Drain Soul and the bar just disappears.. but there's no satisfaction to it. It's empty. That's why I don't play it even though it actually does perform super well.
I find the dotting and then going to town draining to be satisfying enough. Mostly because, thanks to Malefic Grasp, the forced draining as the filler finally managed to not feel pitiful but also because the ability to do damage, improve my damage and heal myself all at the same time gives me that I'm untouchable! feel, at least until I pull too much and they destroy me.
The only reason I made a Warlock was because I wanted to play a dot class again after a long time, so both Destruction and Demonology, with the lesser focus on dots, aren't as appealing as Affliction to me.