FASA Games, Inc.

"Attacking with the weapon requires a full round action for the duration of the spell."

OK, fine for a 5th level fighter. Now it takes him a FRA instead of a standard to attack. Now I am a 20th level fighter and I FRA, do I get one attack or all four? If I get one it is the best spell ever, 2nd level and I gimp all Melee classes to death. If I get all of them it has very little use except to make the melee class run up and then FRA the next round.

I'm pretty certain it is supposed to be a single attack with the weapon would take a FRA. It doesn't seem too OP to me. Consider the spell Slow, which is level 3. It gives multiple targets only a standard or move action (and so your 20th level fighter would get only a single attack or move). Slow Metal Weapon is much less powerful. Especially since to get around the spell all you would need to do is drop your weapon (free action) and draw another. If you think you might run into this spell, bring a backup weapon.

Slow is a good example. I only get a standard or a move. So I can still vital strike, cleave, partial charge and even pounce. With Slow Metal Weapon I can do none of these things because it is a FRA to get a single attack with my weapon. I can 5 foot step and swing. Could you use Spring Attack (as it is a FRA) to still move and attack?

Very good point about being able to swap out weapons, that does drop the power some.

Just noticed that this is a close 20 ft. + 5 ft./2 levels instead of 25 ft. + 5ft./2 levels. Is this departure from the standard range intended?

I don't really want to get into a discussion of which gives you more viable combat options or is more inconvenient, being staggered or needing to take a FRA to make a single attack. If it were up to me, the spell effect should probably read as follows, "the target acts as though under the effects of a slow spell for as long as the target continues to wield the weapon." In terms of the spell level though again, think of the relative power.

Slow = single will save, multiple targets, effects any target, can only be gotten around by dispelling/haste
Slow Metal Weapon = single will save, only one target, target must be using a metal bladed weapon, can easily be removed by dropping the weapon

That tiger over there with only natural weapons, that fire elemental about to set you ablaze, that angry troll with the club, I can slow all of them with one cast of a level 3 spell. I can't do anything to them with Slow Metal Weapon. Honestly, it should probably be a level 1 spell since even hideous laughter seems like a better option as a level 2 spell (sure you get an extra save but if you fail it you can't attack at all and are prone).

I woud tweek your wording a bit to reflect that it only slows you when attacking. "the target acts as though under the effects of a slow spell when attacking with the affected weapon." If reads like that I would agree with it as a 2nd or even 1st level spell.