I never liked the spell thief versions I saw from several sources for one simple reason: they could cast spells on their own. That just didn't have the right flavor to me, so I created my own version that doesn't. Here it is for your critiques.

Spell Thief
Hit Die: d6
Skills: (4 + Int mod) per level
Requirements
Base Attack Bonus: +3
Skills: Knowledge (arcana) 4 ranks, Spellcraft 4 ranks, Use Magic Device 4 ranks, Conc. 4 ranks

Class Skills:
Same as rogue, but add Knowledge (arcana) and Spellcraft.

Saves: Fort: Bad, Ref: Good, Will: Bad
BAB: Medium

Lvl
Ability
1
Absorb Spell
2
School Specialization, 1st
3
Extent Self
4
Turn Spell
5

6
Alter Spell
7
Store Spell
8
School Specialization, 2nd
9

10
Channel Spell

Class Features:
Weapon and Armor Proficiency: Spell Thieves gain no proficiency with any weapon or armor.
Absorb Spell: At level one, the spell thief may ready an action to absorb any spell directed at him or centered on any squares he threatens. To do this, he must first recognize the spell (Spellcraft DC = 15 + spell level) and make an opposed Concentration check versus the spell caster. The spell thief adds his Charisma modifier to this concentration check, as well as his Constitution modifier. If passed, the spell is wasted and has no effect. If failed, the spell takes effect as normal. The spell thief can not pass a Reflex save while attempting to absorb a spell. If, upon passing the Spellcraft check, the spell thief decides not to absorb the spell, he gains a standard action immediately. Absorbing a spell provokes an attack of opportunity.
School Specialization: At level two, the spell thief chooses one school of magic as his specialty. He gains a +2 on both his Spellcraft check and his Concentration check for such spells. Another school specialization may be chosen at level 8. These bonuses do not stack.
Extend Self: At level three, the spell thief may extend his turning and absorbing abilities to a range of 10 ft + 5 ft / 2 lvl (of spell thief). This essentially moves the center of the turning/absorbing check to the square where the spell’s target is, even if that square is occupied. If the attempt to turn/absorb fails, the spell center moves to the spell thief’s square.
Turn Spell: At level four, the spell thief may turn any spell he can absorb. If he passes, the spell is turned back on the caster (or the caster’s square as the center of an area of effect spell). If he fails, the spell thief may attempt to absorb the spell instead. If this fails, the spell acts as normal. Spell resistance applies on the new target(s) as normal. Turning a spell provokes an attack of opportunity. The save DC vs. the turned spell is the original caster’s.
Alter Spell: At level six, the spell thief may alter any spell he can turn into another spell of the same level (including metamagic enhancements) in the school in which he is specialized and cast it using the original caster’s original spell’s save DC at any point within the spell’s range from the spell thief’s location. If he fails, use the chart below to determine how the original spell acts. Altering a spell provokes an opportunity. Spell resistance applies on the new target(s) as normal.
d%
Effect
1-69
Original spell acts at normal at the caster’s intended location.
70-79
A random spell of the same school and level acts on the caster’s intended location.
80-89
A random spell of any school and lower level acts on the caster’s intended location.
90-97
Original spell acts at random location within spell’s range.
98-99
A random spell of the same school and level acts at random location within new spell’s range.
100
A random spell acts at random location within new spell’s range.
Store Spell: At level seven, the spell thief may store any spell he absorbs and use it a number of rounds equal to his Charisma modifier later. No concentration must be held on the spell. If this time expires and the spell has not been cast, the spell fires using the spell thief as the target or as the center of an area of effect spell. Casting the spell at any time provokes an attack of opportunity. Only one spell may be stored at a time.
Channel Spell: At level ten, the spell thief may channel any spell he absorbs into a bonus to one attack roll, damage, ability check, skill check, or save equal to the total spell level. This bonus increases by one if the spell cast is in one of the spell thief’s specialized school. This ability may be stored as a spell, but must be used all at once. Only one spell’s channeled power may be stored at a time.