I like casting summons. The serpents make great air cavalry. They can move across mountains and they can move from sea to shore and vise versa very easily. Furthermore, they have a ranged attack and are immune to melee attacks because they fly. They are great for doing recon and for mopping up and for bringing firepower to an area in a hurry, just like modern army air cavalry.
Imps make good fodder and as some one else pointed out they are great for taking over lairs. They are also kind of like paratroopers because you can summon them behind enemy lines.