By incorporating a number of interesting and varied prestige classes from the 3.0 core rulebooks and supplemental literature, Dungeons & Dragons version 3.5 DMG showcases how characters can be customized and made more compelling to play as and interact with. By more compelling to play as and interact with I mean min-max, twink, and munchkinize.
Nowhere is this more telling that with the spellcasting prestige classes. Long ago when only demihumans could multiclass, it was known that sharing two classes retards growth in either of them, and that this is especially true of the spellcasting classes. Look no further than the Fighter/Mage: At an impressive 20th level, a straight Fighter/mage has 10 levels of Fighter and 10 levels of Wizard. This character would have 10d10+10d4+20*(Con Bonus) hit points, a BAB of +15, access to level 5 arcane spells, six bonus fighter feats, and two bonus metamagic feats. His base saving throw bonuses are +10 Fort / +6 Ref / +10 Will.
Bear in mind that a 20th level Wizard would have a BAB of only +10, 20d4+20*(Con Bonus) hit points, and multiple 9th level spells, of course.
But let’s look at some of the alternate “spice up your spellcaster” options available in the core rules now. A good analogue to the straight Fighter/Mage is the Eldritch Knight, described by the DMG as “studying martial and arcane arts to equal degree.” Let’s see how that really adds up, shall we? To become an Eldritch Knight, the listed prerequisites are proficiency in all martial weapons, and the ability to cast 3rd-level arcane spells. That’s a 1st level fighter/ 5th level Wizard, for those of you taking notes.
Upon taking the first level of Eldritch Knight, this 7th level character has 1d10+1d6+5d4+7*(Con Bonus) hit points, a BAB of +4, casts spells as a 5th level wizard, has two bonus fighter feats, and one bonus metamagic feat. Let’s catch this fellow up with his fighter/mage buddy by adding 10 levels of Eldritch Knight and four more Wizard levels.
A Fighter 1 / Wizard 9 / Eldritch Knight 10 has 1d10+9d4+10d6+20*(Con Bonus) hit points, a BAB of +15, +12 Fort / +6 Ref / +9 Will for saves, two bonus fighter feats, and one bonus metamagic feat. He also casts spells as an 18th level wizard, giving access to mighty 9th level arcane spells such as Wish.
The Base Attack Bonuses match perfectly. The hit points favor the Fighter/Mage five to three. The saving throws are ever-so-slightly better for the Eldritch knight. The Fighter/Mage has superior access to combat feats. The Eldritch Knight can can Meteor Swarm while the Fighter/Mage is still tinkering with Wall of Force.
In the case of the Eldritch Knight, Very little has been sacrificed in spellcasting ability for some substantial martial benefits above the straight Wizard route. 50% better base attack bonus, far better hit points, and superior fortitude saving throws at the cost of two spellcaster levels of arcane firepower and three bonus metamagic feats. The advantages over a straight Fighter would appear obvious: 18 levels of spellcaster firepower in exchange for 9 bonus fighter feats seems a fair trade (have you ever seen Great Cleave put to as much use as Fireball? Neither have I). In comparison to the Fighter/Mage it isn’t quite as clear, as both spellcasting potency and a great number of Fighter feats have been sacrificed along with the all-important hit point edge of Fighters.
Clearly each has its benefits, and depending upon your character and your campaign’s story arc, you could go either way. I dare say that the Eldritch Knight has a serious advantage, but this is a matter that reasonable people can reasonably disagree on.