I think that the Forcemaster in general has some extremely powerful ways to effect what she wants on the battlefield (including Thoughtspore) and is greatly rewarded, even over other mages, for out-thinking your opponent. If you can correctly ascertain what your opponent's overall strategy is, your book should have the answer for it. For instance, it's a terrible idea to put a sleep on your Thoughtspore on turn 1 for the very reason that Schwenkgott says. However, if you see a Lair go down or a big bad creature, then yes sleep is a decent choice.
All that to say that the best Forcemaster is going to drive toward the enemy correctly guessing what he/she will do in response and having a counter already prepared. She's a really rewarding mage to play well.
I've got a best and worst for my Thoughtspore antics:
Best: Acid Ball vs. an armor stacker. To be fair...he should have put more ranged attacks in his book
Worst: Seeking Dispel. I never even cast it. The only good that came out of it was that my opponent was scared to death what I might need all that mana for.