Expert skills are subjective. They have different values for different parties.
Strength (Powerstrike): Useful for anybody who uses melee weapons, whether close range or extended range. Anybody who does even a moderate amount of fighting should strive for this, except of course, primary magic users.
Intelligence (Powercast): Useful for anybody with magic, completely useless for the Fighter, Rogue, Bard, or Gadgeteer. Bards and Gadgeteers "use" items (instruments/gadgets), and don't actually cast.
Piety (Iron Will): Useful for anybody, as not only does it increase base resistance, but helps against side effects of physical attacks (Savant Minions in Arnika with extended range, paralyzing polearms). However, it is very costly to get for characters with no magic.
Vitality (Iron Skin): Useful if one character is going to be tanking a lot, otherwise, not so much. A few points of damage resistance pales in comparison to what Body of Stone can do. Not to mention, you actually have to be hit for it to count towards a skill increase.
Dexterity (Reflextion): It gives a moderate boost to AC, though the benefits of having high Dexterity are more important than this expert skill.
Speed (Snakespeed): Having a high base Speed (before items/spells/etc) is the best (Underwater/Unlucky Bard casting of Haste instrument sucks; You can't control which power level the Bard casts). Snakespeed allows you to act first, especially important for casters who intend to cast Soul Shield/Element shield to protect the party first. Also, moving first is always good.
Senses (Eagle Eye): Useful for anybody with a ranged weapon. Even casters can greatly benefit from this skill (Especially the Psionic and Mage, who have no useful melee options).