It all depends upon how you structure your multiclass.
If you are a figher/sorcerer and are envisioning your character as such, then you are not expecting to be able to blow away every creature you encounter with your amazing attacks or your amazing 9th level spells. In fact, you are limited to 5th level spells and you have a lower BAB and fewer feats. If however, you build the character with that in mind and take lots of "buffing" spells, then you can make your character perform better than he would otherwise appear to. Also remember that a fighter/wiz or sor 10/10 has the magical items of a 20th level character. Just think of this:
A figher sorcerer with some of the following spells/magical items
True Strike (a +20 upwards of 6 times per day can be handy and with a ring of wizardry this can be even more)
Haste (this seems like a no brainer).
Bull's Strength, Cat's Grace, Endurance (again, very handy)
Improved Invisiblity
Blur
Any of the touch spells (one of your fighter feats on Improved Unarmed Strike and you avoid that nasty AOO)
The raw damage spells for a first strike
Fly
Darkness (if you have blind fight)
Blink
With bracers of armor, ring of wizardry, weapons of spell storing, and the craft magical arms/armor feat, you can be a potent oppenent who uses his magic to suppliment his fighting rather than trying to be the equal of a either a fighter or a spellcaster. Even without haste, this character could cast True Strike (even metamagicked if need be), adjust 5' forward or more) to his foe, attack the next round, (at a +20) on the first one and then move back. That way no pesky AOO for spellcasting. With haste, he is in even better shape.
One of the areas that weakens multiclassed spellcasters at high levels is the spell resistance of their enemies. The solution is to use spells that either affect the good guys or manage to circumvent SR (not many of those).
The problems with the multiclass system only come out when a person remembers how powerful their fighter 10/wizard 10 was in 2e compared to their compatriots and thinks that it can be exactly replicated in 3e. It can't. The only reason to ever multiclass in a spellcasting class is if you want to use the spellcasting class as a complementary class for the other (excepting the 1 level in fighter type of thing). Only a single classed spellcaster can pull off the major attack spells against creatures of their CR but that is really the only way in which they are superior.
-DC
who actually has a sorcerer / rogue in his current campaign. Her player was very frustrated at her inability to affect enemies with her attack spells. It finally occured to her that as long as she kept leveling up in rogue without taking "buffing" spells, she would be frustrated. Finally she opted to start leaving her roguing ways behind and focus on her sorcerer stuff...it was the only solution that kept-up the concept of the character (she'd started as a rogue and dicovered her sorcerous powers later; she ended up embracing the new lifestyle completely).
