Typically, dual classing is great when you go from a high HD class with great early level capabilities (Fighter, Ranger, even Cleric to a lesser extent) into a low HD class that sucks early on but has better high level scaling (Mage, Cleric, and to a lesser extent Thief, but only relative to pure Fighter). As long as you dual class out before XP/level scaling switches to linear, you are missing out on at most 1 level compared to a single-class character. Heck, the XP cost of going up to the end of HDs for most classes means you wouldn't even be a full level behind if you dual class into Mage.
Fighter 9->Mage gets about 50% more HP than a single-classed Mage (with 16 Con, more with higher Con), at the cost of just over 3/4 of a level (250k/325k, not even a full level). Sword swinging action, elven chain, better PPD and Breath saves, and expanded magic item options are just a bonus. Fighter 3->Mage costs a negligible 4k XP (just over 1.2% of a level beyond 11th) for 9 additional HP (about a 20% boost over straight Mage, about the same as multiclassing) and those same sword, armor, and magic item options, while also helping you through a significant part of those painful first levels as a Mage (you still won't contribute, but at least you won't die to a stiff breeze) and letting you feel the Fighters power at the levels the class is strongest. Fighter 7->Mage optimizes the dual classing time for that extra 1/2 attack and the sweet spot where Mage levels up really fast for a couple of levels, IIRC, so you get your Fighter kit back faster.
Fighter 7->Thief gives you no small chunk of HP (about 25% more), costs only about 1/2 of a Thief level, lets you use actual armor when you don't need Thief skills, and lets you do crazy damage with weapon specialization and extra attacks if you can get a backstab situation off. Thief saves kinda suck, IIRC, so you're getting a nice boost there for a while. Also, Thief levels up quickly, so you don't spend much time without your Fighter kit.
Fighter 7->Cleric gives you a bit of HP, but mostly lets you become a combat machine with extra attacks multiplying those Cleric self-buff spells that I'm pretty sure existed outside of Baldur's Gate and the rest of the infinity engine games. XP cost is, again, about 1/2 a Cleric level once you're into linear scaling. IIRC, it also hits just before Cleric leveling hits a speed bump for a bit, so you get your Fighter kit back soonish.
Thief->Mage is a toolbox thing. You get thief skills, a bit more HP, and a slightly better low-level experience (bows are OP), and the XP cost is really tiny in the long run. Thief levels up fast, though, while Mage levels up slowly, so you do spend a relatively large amount of time as an underlevel single-class character.
Multi-classing is a trap. It's decent enough at level 1 (when you get something for nothing) and at levels 4-9 (when you get decent versatility at the cost of not too much power). At high levels it sucks, as you're advancing at half rate in whichever one spellcasting class you really want to scale up, you have less HP than a dual classer would have, and if you're using level limits for non-humans, you're going to hit them because you can't be human.