The way I read it, a Wiz1/Ftr4 would have the spells per day of a Wiz1, but could cast those spells as if he were a Wiz5, giving him better effects/durations.
Is this overpowered? Why? Why not?
You're a 5th level character with the spells per day of a 1st level wizard. Some spells (e.g. sleep, cause fear, command) don't do much at 5th level power. Really, the feat's best used by gishes to keep up their caster level for boosting durations and to occasionally bypass SR. I've seen some good use by fighter/clerics as well.
I think it's an important feat for a lot of multiclass casters to take and that's the main problem. You never really want any single feat to be a complete no brainer. Everything should have some trade-off associated with it, some opportunity cost. This feat runs very close to the no-brainer territory.
But I don't completely object to it because, in many ways, it brings a little more balance to a trade-off that was probably too steep in the initial rules: the loss of a spellcaster level for every non-spellcasting class level you take. And when it comes to calculating spell damage, spell duration, resisting attempts to dispel, and overcoming SR, spell level is very important.
I first realized just how much a loss of spellcasting levels could be when I read some playtest notes for City of the Spider Queen posted by James Wyatt. Clearly, Savage Species had recently been tested/worked on/published because a lot of the characters were monster-types with ECLs. While they still worked fine for the fighter types since the ECLs tended to push high based on monster strength and size, spellcasters were looking very weak. When the adventure got to higher levels and the hill giant cleric was an 8th level caster who could hardly penetrate a drow's spell resistance, you could see something was up. The same can hold true for multiclass spellcasters without ECLs. Eventually, you're dealing with creatures whose defenses are geared toward a single-class caster's ability to penetrate and having trouble stepping up to help.
So concentrate on the buffs, you say? You can do that, but it's still nice to have a few offensive spells to cast from time to time, to mix up the strategies a bit and keep the game fresher.
So, in the end, the feat is a very powerful feat as far as individual feat vs feat comparisons go. But it's a very useful patch to the system.
Good points. Overcoming SR is the big bugaboo the multiclassers deal with.
A Mystic Theurge with a CL of 17 isn't going to be really good at that at 20th. If he takes the feat (twice, in his case), he only deals with the lack of high level slots for his trade-off.
The Arcane Trickster and Eldritch Knight could use this feat, too, as well as the fighter/cleric combo mentioned.
A decent "patch," as you said, even if it screams no-brainer!
Other opinions?