If each Extra Spell gave you an additional known spell of each level you can cast ... and was updated as you went up levels ... it owuld be, IMO, worth it at any Sorceror level.
Or perhaps if it allowed you to (with your DM's approval, ofc) add a single spell form a DIFFERENT class' spell list. Maybe.
As it is, though ... I'm all to easily able to come up with alternate lists of feats I'd much rather get. For example, if going for the archtypal "artillery sorceror":
Ch-1 Empower Spell
Ch-3 Elem Substitute, Fire
Ch-6 Elem Substitute, Ice
Ch-9 Elem Substitute, Acid
Ch-12 Elem Substitute, Electric
Ch-15 Elem Substitute, Sonic
Ch-18 Maximise Spell
Combined with knowing the 1st level spell (form MoF) "Know Protections", or similar, you can know "which element are you weak to, today" ... and use it. The spell list for the above would be very heavy on elemental-tagged damaging spells (Burning hands in preference to Magic Missile, for example).
But Extra Spell?
Not even for THE most-useful-spell-I-haven't-already-picked. Ever. Under any circumstances, as the feat is currently written. Its a neat idea, and parallelled the Wizard-oriented Extra Casting, but parallelled it TOO closely, making for an over-weak feat.
IMO, of course.