I played Volker.
I didn't have the water path (affinity 0). One really huge problem as an invoker is that many of your elemental spells count not purely as elemental damage, but *magic* elemental damage - that is, dwarf resist_magic will reduce the damage. This is true for pillar, forked, wall of fire, fireball, etc. Iceneedles is purely piercing, no magic. Blackfire nova, a quest spell, is fire/negative, no magic (which is why it was one of my favourites against resist_magic races). Rain of stones is blunt, no magic, earthquake is blunt, maybe no magic too.
This is another one of those really obscure bits of information that took me ages to figure out. I was wondering why I hit resist_magic races for so little damage with pillar, but rain of stone was doing straight DEMOs (no preps on the dwarf).
Invokers are really powerful, but not in the super big damage way everyone seems to think they are. They're powerful because they can get 90+% damage reduction, have a huge variety of spells for almost any situation, have great survivability and utility. They're not very deathful, except in big group fights where shield of electricity, rain of stone, quicksand, conglaciation can wreak havoc. 1v1, I'd rather be a bash spec warrior with a secondary lagging spec. They are really good for making people run away though. Just put up ABS/st, and make everyone run.
The other thing was save vs spell. People with huge saves did not merely take half damage from my spells - they took anything from a quarter to half damage. Damage was absolutely miserable against people who had good saves (bards, paladins), even taking into account their damage reduction. I'd say saves are as important against invokers as they are against necros. Some spells do bypass saves, such as iceneedles. I'm not sure about earthquake, earth ripple, rain.