does spell penetration do any good for druids?

Depends.  Are you doing caster DPS, healing, feral DPS, tanking, or what?  For caster DPS, it's definitely good.  For healing, it's worthless.  I don't think it affects your abilities in cat or bear form, but I could be wrong there.

Spell penetration is worthless for pve encounters. It is for pvp and as such does work if you have damaging spells used on other players.

Spell Penetration is like Armor Penetration for melee dps, just that it's worthless in PvE.

SpellPen = PvP, 'nuff said