Multiclassing can be advantageous for fighter type characters.
After a certain point, many fighters have enough feats to be effective. The abilities of barbarians, rangers, and paladins are then attractive options. Similarly, a feat starved barbarian doesn't sacrifice too much (2 hit points, 4 skill points, uncanny dodge progression, rage, and DR progression) by picking up two levels of fighter. Any lightly armored fighter or barbarian can pick up a level of ranger to dual wield, track, use wands of cure light wounds and get ranks in spot and listen--in effect, they're giving up one feat for two virtual feats, one real feat, expanded magic options, and a good selection of class skills...
Multiclassing spellcasting and non-spellcasting classes can be effective but is generally a lower powered option than staying single classed as a spellcaster (or avoiding spellcasting entirely). A fighter/cleric, rogue/wizard, or fighter/wizard can be made effective if the player is willing to focus on the character's strengths and make them good at what they do. Even so, they will often be inferior to single-classed characters of their level. (I play a melee focussed Fighter 2/Wizard 4 in the RPGA's Living Greyhawk campaign--he's not as good at dealing damage as the really highly optimized characters but he seems to be about as effective as the average fighter of his level).