So I was looking at the feat burning spell:
Burning Spell (Metamagic)
You cause creatures to take extra damage when you affect them with a spell that has the acid or fire descriptor.
Benefit: The acid or fire effects of the affected spell adhere to the creature, causing more damage the next round. When a creature takes acid or fire damage from the affected spell, that creature takes damage equal to 2x the spell’s actual level at the start of its next turn. The damage is acid or fire, as determined by the spell’s descriptor. If a burning spell has both the fire and acid descriptor, the caster chooses what kind of damage is dealt by the burning spell effect.
Level Increase: +2 (a burning spell uses up a slot two levels higher than the spell’s actual level.)
Now let's say I use this feat when casting a fireball. The description would lead be to believe that the only damage I would gain would be 6 at the beginning of the next round (2x the spells actual level: 3) But that just seems like such an awful reward. Much worse than empower spell which does the extra damage immediately, and it's more damage, and it's not specific to an energy type. So I'm hoping that I have the description wrong. How can this feat even exist if it's as useless as i think it is?
Well if that is the full text of the feat then yeah, looks like 6 additional damage on a fireball. So think of it like an extra die, i suppose? It really doesn't seem worth a 2 level increase to me. I guess it would be alright (mediocre) for area damage from the fireball, 6 damage hitting all targets (if there were a lot). I wouldn't take it over empower or the like. You only need do 12 damage for empower to be equal.
Best use of the feat would probably be putting it on a spell that does some damage over time or sets the target on fire (might be iffy on RAW for that, but if the spell said "catch on fire" I'd roll that into being caused by the spell for the feat). You would be getting the extra damage every round of the effect. It would also potentially allow for fire/acid resistant opponents to take damage from those spells (1d6 a round would hardly ever hurt a resist 5 opponent, but 1d6+ 4/6/8 would).
A lot of the metamagic feats from that book seem a bit off, but I agree that this one seems particularly weak. You really shouldn't have to go out of your way to find a use for a feat (and it's not even a feat tax for other, better feats).
I'd agree with Skylancer on this one. Might be useful with something like Acid Arrow or Acid Pit where damage is taken every round.
I've used it to GREAT effect with wall of fire, fire shield, acid arrow and similar ongoing spells.
You'd be surprised how much damage one can do with it when you put it on the proper spells.
I suppose it does better than empower spell on high-level/low-damage-over-time spells, like wall of fire.
Also, isn't that continuous damage damage for a round? That means casters have to make concentration checks the following round. 10 + 1/2 damage dealt + spell level isn't very difficult, but if you have other class features and feats that boost that damage it could add up.
I agree that it doesn't seem as good as Empower.