I'm playing a Cleric right now an I have Spell Focus(Necromancy). I am level 6 with +6 to Wis [Periapt of Wisdom +3 (homebrew item) & 19 base]. I have convinced my Dm to allow retraining of 1 feat/level of any that was applicable at the time the original was taken.
So my question is should I go with Shadow Weave Magic since it will mostly be all beneficial or is the +1 DC to spells level 0-3 with DCs of 16-19 worth it? Just to clarify, is Spell Focus in general worth it and does it become worth it if you burn 3 feats to increase DC by 3?
Well first off, Shadow Weave Magic will lose you two points of Wisdom until you've jumped through a bunch of hoops to get them back. That doesn't seem like an ideal situation for a Cleric to be in, but I guess if your DM is prepared to hand-wave that away, fair enough.
How many saving-throw-allowed spells from Enchantment, Illusion or Necromancy are you casting? If the answer is "lots" (and you're not casting a lot of Evocation or Transmutation spells, which take a penalty) then it might be worth it. But as Empirate says, it might be
more worth considering whether you actually want to be casting those spells in the first place.
In general, I've only taken Spell Focus when I've got a specialist Wizard, who will then take the Master Specialist PrC and get Greater Spell Focus for free, unless I've got something more interesting in mind. I don't usually bother with spells that allow saving throws, but there are a few Conjuration spells (
glitterdust FTW!) that can make Spell Focus (Conjuration) worthwhile. That's assuming your build isn't feat-starved: if it is, forget it.
Clerics are a different kettle of fish. You've potentially got access to Divine Metamagic [Heighten Spell], which is going to beat the hell out of a +1 boost to your lower-level spells in the long run. They're generally not big on Illusions, so you won't see a lot of benefit there. To be sure, they get some nice Enchantment spells, but nothing you can't live without...
Unless there's some sort of over-arching concept thingy happening here (or else some cheesetastic abuse I'm missing) I don't really see Shadow Weave Magic or Spell Focus being worth it for you. Arcane casters get better mileage, by and large.
EDIT: Ninja'd by the OP... that'll teach me to wander off halfway through writing a reply!