nordetsa wrote: Anyway, is it worth investing charm skill for that? I figure I would have to decided between prism and arcane familiar for my second high-level spells (IMB would be the first one).
IMF feels a lot like Sticky flame. Amazingly MP-efficient, good at killing most enemies through Lair+Orc, and slowly dropping off after that. Prism feels a bit like Fireball; useful on hordes, when you don't mind waking everyone up. As that is partly covered by IMB, I would definitely take IMF. In fact, I'd probably take it before IMB, but that's pure preference. IMF will let you usually two-shot Ogres with Magic dart, and one-shot Orc priests and wizards. IMB will also let you do that, but for a higher mana output, and lower time investment.
Anyway, you'll eventually be wanting Charms on any caster. Hexes are optional, though nice.
When would be best to move to other elemental branches?
When you get good higher level nukes. If you're worshiping Veh, consider grabbing a low-level one of the two skills you're interested in, and training them to 3-4 to encourage useful gifting. If worshiping someone else, when you find a good book. Maybe the bottom of Lair or Orc; your starting book is plenty fine for that.