Intro
Metamagic is meant to allow casters to perform 'tricks' with their spells. In general, (*spell* + *metamagic*) is more powerful than (*spell*).
Metamagic feats were tacked on in third edition. WotC tried to apply linear benefit (metamagic feats) to an exponential system (spell slot value). Most metamagic feats aren't like a Fighter feat that's always on (Improved Initiative) or a feat that's easily activated (Power Attack). There's always an additional cost.
Extra Costs
In general, metamagicked spells come too late to be useful. If you could Extend mage armor from level 1 (making it last 2 hours) or ray of enfeeblement (making it last 2 rounds), then it would be a serious candidate as a level 1 feat.
As it is, you probably don't need the duration boost once you could legally apply Extend Spell, and Extend is considered one of the more useful core metamagics.
Alternatively, let's say you took the feat Empower Spell. It has no feat, skill, or stat prereqs, and some Wizard may have even taken it at level 1.
If you're a prepared caster, you must declare you're using it by using a spell slot 2 levels higher than normal. If you're a spontaneous caster, you're almost always using a small number of spells known and must use a longer time to cast the spell. (I know there are ways to avoid the extra time, or turn prepared casters into spontaneous casters, but those are resource trades or further resource investments.)
Excluding fast progression casting classes, the soonest you can use it is level 3 to Empower a cantrip. Even blasters realize that an Empowered acid splash is only (d3 * 1.5) while a scorching ray is 4d6.
In an 'optimal' case, a Wizard9 or Wizard10 can do (9d6 * 1.5) or (10d6 * 1.5) with a level 5 slot compared to a normal level 5 spell which would probably only deal 9d6 or 10d6 damage.
Metamagic Rods
Metamagic rods are core, and for good reason. The designers subtly said, "Oops!" and included a quick means to rectify their system.
My Experience
I've played 2 Wizards in epic (level 30+), one Wizard from level 1 to ~18, another Wizard at 21, and another Wizard under a variant spell point system who's level 10.
My level 18+ Wizards only used metamagic when it was spontaneously applicable or free. Among the 3 Wizards, I may have cast 10 metamagicked spells this way that weren't Auto-Quickened.
The only reason my level 10 Wizard uses metamagic is because I can spontaneously apply it, and I pay in spell points, not spell slots. Also, the DM lets me spend as many spell points as I have, meaning I could cast a level 5 spell with 10 metamagics if I had the feats and spell points. In sum, I can use my metamagic feats, and it's fun!
The Dilemma
We're stuck with a dilemma. What do we do about metamagic feats? Casters are already powerful, and giving 'free' metamagic would uberize casters even more.
Yet it does not seem fair to recommend or require a bunch of useless feats to anyone. My precious resource (feats) had better get me something worth the investment. It also isn't like Fighters can only Power Attack with their secondary attacks, which metamagic feats seem to do to spellcasters.
Questions I never had answered to my satisfaction: Which spells are we meant to metamagic? How powerful are these meant to be overall?
Alternative Systems
For those displeased with the current system and in the DM's chair, consider these options.
1: At cast time, allow a skill check to reduce the metamagic's cost. Similar to a 3.5 Incantatrix, this allows those blessed with epic Spellcraft (or a skill of the DM's choice) to, y'know, use their metamagic in a friendly fashion. Passing the check means the metamagic is free (woohoo!) or cast at a reduced cost (yay).
If using the reduced cost option, also consider option 2.
2: Force casters to give up a spell level equal to the metamagic slot increase. Adding Empower Spell (+2 slot levels) to a spell would require you expend a level 2 spell slot as well as the spell you were originally casting. Adding Extend (+1 slot level) and Maximize (+3 slot levels) would require losing a level 1 slot and a level 3 slot.
Alternatively, you could spend a number of slot levels equal to the total cost. Adding Quicken (+4 slot levels) would require spending 4 levels worth of spells, such as 1 level 2 and 2 level 1s.
3: Nix metamagic feats and make metamagic 'rod only'. Many players already avoid metamagic feats. Now, this would officially apply to everyone.
Since many metamagic feats lack an equivalent rod, you would need to determine such an item's price.
4: Allow metamagic feats to give a certain number of free daily applications. For example, taking Extend Spell gets you 1 free daily application for any spell you know.
Perhaps you could retake Extend Spell, getting 2 more free daily applications each time.
What I Would Do
I believe metamagic feats are meant to make your lower-level spells comparable in power to your max level spells.
1: Nix Heighten Spell and remove it as a prerequisite. Instead, a spell cast from a slot level is treated in all ways as a spell of that spell level. Normally, an Extended acid arrow requires a level 3 slot but is treated as a level 2 spell. Now, it's considered a level 3 spell in all regards.
The existence of Heighten Spell creates unintended synergies by the rules as written. Shadowcraft Mages and early entry tricks attest to how this feat is typically used.
2: Allow free metamagic applications to lower level spells. Let's assume you're a Wizard9 with Extend (+1 slot level), Empower (+2 slot levels), Maximize (+3 slot levels), and Quicken Spell (+4 slot levels). You can freely apply Maximize Spell to your level 0-2 spells, since they don't require a spell slot of a level you lack. You can Empower and Extend a mirror image as it would normally be a level 5 spell. Likewise, you can Quicken a shield.
Best of all, these spells don't require higher level slots!
If they did, we'd be back to the core rules with spells that cost more (feat + spell slot) but do less.
3: Rebalance existing metamagic feats. Persistent Spell (+6 slot levels) is a joke. A level 13 Wizard who can plane shift and who may have already taken over the world isn't paying using a level 7 slot to make his shield last all day.
Metamagic feats were a sly way to pad book length. It looks like it has potential, but it ain't worth it by the time you get there.
The 'Big' Metamagics: Extend, Persistent, and Quicken Spell
People take these to save actions. Really. If I want something to happen now, I would have Persisted it to make it last 24 hours, or would Quicken it so I can take the rest of my turn.
Extend Spell is Persistent's little brother. Extend is rarely worth a feat on its own, but Extend + Persist = buff joy.
Rarely have any other metamagic feats been generally recommended. Blasters may take Empower and Maximize, crowd controllers may take Sculpt, and buffers may take Ocular and Chain. Regardless, Incantatrix or Divine Metamagic are usually involved.
Persistent Spell - Reconcilable?
Persistent Spell is in an awkward position. For players and DMs, it means you know a buff will be active for a day unless it's dispelled or an antimagic field envelops the target. Persisted buffs are usually uber because they change potent spells from a fight or few (fire shield, divine power) into something that's on all day.
Many have tried to price Persistent Spell based on the original spell's duration. A round/level spell would cost more than a 10 minute/level spell. I understand why you'd do this, but it just feels wrong to me.
This dilemma exposes another problem with the system: A round/level buff requiring a standard action to cast is almost never worth casting in combat. By using such a buff, you probably put your allies in greater danger because you did nothing to directly hamper the enemy this round.
Such short duration spells expose something about player attitudes, at least in my experience. Every round, players want to use their action offensively. Casting haste in combat on a physical-heavy party is like shaking up a can of soda in a rocket-powered paint mixer. Conversely, casting displacement in combat is asking the enemy to pound on your allies instead, perhaps in a disproportionate manner.
