My take on it:
Spell focus is no longer a good feat. In fact, at +1 DC, it's a lousy feat. However, it's still a "must-have" feat for any spellcaster who specializes in one school of magic.
Why is this important? Wizards and sorcerors live or die by their save DCs. A 10th level fireball does an average of 35 points of damage if the save is failed and 17 if the save succeeds. 35 points of damage is signifcant at that level; 17 points of damage usually isn't. It's even worse for wizards using will or fort save spells since foes who make their saves usually experience no ill effect from the spell.
How can a feat that grants +1 to DCs be lousy? Well, to begin with, it will rarely apply to more than 50% of the spells a wizard casts. Let's say a typical 5th level evoker has the following spells prepared:
1. Magic Missile x2, mage Armor, Shield 2. Scorching Ray x2, mirror image, flaming sphere 3. Fireball x2, Slow
This wizard is very focussed on direct damage spells--to the point that some would say he's quite vulnerable if he runs into fire resistant or spell resistant creatures. However, his spell focus feats would still apply only to three out of his 12 spells. Wizards who take fewer spells without saves (magic missile and scorching ray don't have saves yet comprise one third of his spell selection), might be able to get that number up to half.
Now, +1 DC is significant if you're tossing a spell against an opponent's bad save. For an 8th level fighter with a +5 reflex save, the difference between a DC 17 and a DC 18 is about a 13% lower chance of making the save. However, spellcasting characters are rarely afforded the luxury of casting spells against their opponents' weak saves. Usually, they're forced to spend a significant amount of time tossing spells at their foes strong saves. When the foe has a reflex save of +11, the difference between a DC 17 and a DC 18 is about a 6.7% lower chance of making the save. Not nearly so significant.
So, how can a lousy feat still be "Must Have"? For any character focussed on a particular subset of spells, any possible DC increase is necessary. If the evoker isn't to be useless when facing rogues and the enchanter isn't to be useless when facing clerics, he'll need to pump his DCs to the point that +11 reflex doesn't mean automatic success. (And at level 10-12, foes' weak saves will often be +11 so if he's to have any hope of affecting any foes with his lower level save-permitting spells, he'll have to have Spell Focus, GSF, and a high starting int).
Characters can still try to be generalists and get by without spell focus or greater spell focus but with the saving throw boosts given to monsters in 3.5, monsters will usually succeed even in their weak saves. And the difference between a 10% and a 20% chance of effecting a monster is significant.
Spell focus and Greater spell focus no longer make spell casters good--they are able however, to keep them from being utterly worthless.