How big an issue is spell resistance?

[CyberThread]

I don't normally play spell casters, ** I tend to play on play by post not local groups **; so no consistent monster choices or DM picks, but how big an issue does spell resistance go for Casters

An how useful would it be for me as a sneaker to get the feat Undo Resistance , to aid casters in whatever they may be doing?

[Biffoniacus_Furiou]

Most of the best spells, whether blasting or crowd control, are from the Conjuration school. Nearly every one of those already gets to completely ignore spell resistance anyway. If a caster is using SR: Yes spells, then there are already spells like Assay Spell Resistance and Spell Vulnerability to help them out when they need it.

[Jeff the Green]

It depends on the caster, really. If we're talking an optimized wizard or sorcerer, it's pretty irrelevant, as Biffoniacus_Furiou said. On the other hand, if it's a beguiler, warmage, or dread necromancer or an unoptimized caster, it can be pretty irritating.