I've replaced the voidmage prodigies maindecked fire/ice. There are a bunch of Fish decks and Workshop decks in my meta, and fire/ice is strong against both of these. In the mirror, Fire is clearly a good card, and is un-misdirectionable in most cases. Against workshop decks, Fire can kill welders and metalworkers. In addition, I sometimes use Ice to slow down the Workshop player early on, in order to get a standstill "lock" before spheres of resistance and wastelands become too much of a problem.

They are useful against aggro, too - better in most aggro matchups than a Prodigy in my opinion. The counter-ability rarely comes into play because it's so slow in this case, so the mage usually just chump blocks and dies. Fire/Ice isn't incredible either, but it's never a dead card. Fish usually has a pretty good matchup against pure control decks with all of the mana denial and manlands, so the change from Voidmage to Fire/Ice isn't as curcial in this case.

Are Trinisphere and Sphere of Resistance cumulative? For example, if one of each sphere is in play, would an Ancestral Recall cost 3 or 4 mana to play?

For reference:

Trinisphere -  
Artifact (R)
As long as Trinisphere is untapped, each spell that would cost less than three mana to play costs three mana to play. (Additional mana in the cost may be paid with any color of mana or colorless mana. For example, a spell that would cost 1B to play costs 2B to play instead.)

Sphere of Resistance
Color= Artifact  Type= Artifact  Cost= 2 EX(R)  
Text (EX): Spells cost more to play. [Oracle 1999/05/01]
It affects all players, including you. [DeLaney 1998/06/10]