Extent Spell: It's only nice with the right concept and within the right level range, so all in all not too great. The Favored Soul character I've recently written up has Extend Spell and it's probably the best choice for her. But that character is also at the perfect level (12th).
Silent Spell: For many caster this is essentially an extra level, since you lose one, when you get resurrected.
Quicken Spell: +2 levels!? You must play a different game, than I do.
Even with +4 levels it is mighty powerful. At higher levels, of course (11/12+).
Empower Spell: Highly potent as has been pointed out enough already. Better than Maximize IMHO.
My Sorceress currently has Silent Spell (was useful quite a few times already, and it it's useful, it's
really useful) and Empower Spell (is used multiple times each day normally). Next level (Sorcerer 6th / Incantatrix 4th) I'll get Heighten Spell, which will probably see as much use as Empower.
After that, it grows a bit weaker. Quicken Spell is the next, but as an Incantatrix and later with Arcane Preparation, this can be put to good use, even if it cannot be used normally. Even with that restriction, it will be probably one of the most powerful metamagic feats for my character! After that, Twin Spell, or maybe Chain Spell (hope there will be a clarified and better working version in Complete Arcane).
And I'm constantly looking for a metamagic rod of energy substitution. Having only fire-based energy spells (Scorching Ray and Fireball) is a bit of a disadvantage. Altho, she is not really an evoker (prohibited school Evocation since 7th).
For wizards, I don't think there are so many metamagic feats, that would be used a lot. Empower and Quicken mostly, and Silent Spell is worth it for the emergency Dimension Door alone.
Bye
Thanee