Spell Combat (Ex): At 2nd level, a magus learns to cast spells and wield his weapons effectively at the same time. This functions much like two-weapon fighting, but the off-hand weapon is a spell that is being cast. To use this ability, the magus must have one hand free, while wielding a light or one-handed melee weapon in the other hand. As a full-round action, he can make all of his attacks with his melee weapon at a -4 penalty. He can also cast any spell with a casting time of 1 standard action from the magus spell list. He must cast this spell defensively, and he takes a -2 penalty on the concentration check. If the check fails, the spell is wasted, but the attacks still suffer the -4 penalty. A magus can choose to cast the spell first or make the weapon attacks first, but he cannot cast the spell between weapon attacks if he has more than one attack. Casting a spell in this manner does not provoke an attack of opportunity. The magus must have one hand free to use this ability, even if the spell being cast does not contain somatic components.
What are your opinions on this ability?
My main gripe with it is the -4 penalty. Sure, it's a powerful ability, but a -4 penalty to hit on a medium BAB class is far too steep, at least early on. A 2nd level Magus with 16 STR and 16 INT fighting a creature with 15 AC will have a roughly 25% chance of hitting the target with his weapon and a 30% chance of casting a 1st level spell (roughly 50% with combat casting). Not exactly reliable. This becomes even more bothersome when you factor in that it requires a full-round action.
I'd prefer it to work pretty much exactly like two-weapon fighting, with the spell being cast being treated like a light weapon. The two-weapon fighting feat would reduce the penalties as it does normally, so that you could reduce the penalty down to -2.