From a high-level perspective, I played through to the end of Act I as a mage (with another mage partymate, plus a rogue) without spending any Source points.
The higher memory slot cost balanced against the (unknown) availability of Source points made less attractive the Source abilities I did bother learning. Source abilities (aside from Bless, which doesn't use a Source points) didn't have a huge draw that made them irresistible. That said, in the greater campaign to come, with more exposure, it's likely that I'd end up using Source points more.
Good scouting, resultant tactics and normal (lol) magic were the path I chose.
