+1 to the strobist blog. You'll learn a lot from it, even go full manual on your flash setup. I use TTL most of the time when flash is on-camera but if I want to be extra creative, I use full manual off-camera. You can control the shadows and where would you want to put your light or how strong you want your flash this way. Just refer to the pics you'll find in the blog to know what I mean.
One of the best pieces of advice I've heard so far, with regard to TTL/ETTL...is from Syl Arena. He says he only uses ETTL, when the camera lens are subject are generally moving and changing distances from each other. Which kind of makes sense.
If run and gun, it helps..but if you're taking time to set up shots, there's not much need for it...manual is the way to go.