I'll also add that, while basic Shem's isn't anything special, the boosted version brings a long-range, high-Strength attack, filling in our lack of war machines (no offence, Giant Bow...it's just that you always miss).
Net seems to me like an awesome anti-magic tool. Most Wizards won't have a high Strength, and they'll need to pass a Strength test for every cast. And, if I'm not mistaken, they actually have to roll the power dice first (as it says "casts spells", not "attempts to cast spells"), and then pass a Strength test. You could wipe out half an enemy Wizard's magic, and potentially waste a whole lot of his dice.
A question, though: You cast Net of Amyntok on a unit that has S4, joined by a Wizard with S3. He casts a spell. Does the opponent take the Net test on the Wizard's S3, or the unit's S4?