I was thinking about this question while answering an earlier post; Which is better: a single class char or a dual-class one?
Personally i prefer single class chars because they go up in thier skill levels far faster. Example being if you are a mage then one level up you gain better saving throws and the next you get better spells, if you dual class then this process takes far longer. Anouther disadvantage to dual class chars that i see is the amount of restrictions that your char will have. If one of your classes can't have aromor than bouth cant have aromor,I know that they have a further spread of abilities but i belive the restrictions far outwhiegh the benifits. Maybe it's because I am new to RPing or that I am a purist at heart but Single class chars got the upper hand IMOHP.

I prefer single class if only for the sorcerer my all time favorite char.

I am playin with a dual classed fighter to a cleric at the moment. As far as the fighter side go, you'll loose on some things but as far as the cleric side goes. It's much better. More HP then a full cleric. Better thAC0 and GM in a weapon class (warhammer for me). Besides the gaining of levels isn't that big of a deal. You'll breeze through the lower levels. I did a quest (don't remember which one but it gave quest exp) and went from a 2nd or 3rd level cleric to a 6th level one. You have to be a bit patient though to reap the benifits but dual classed char. can be very powerfull. Every dual class has its benefits and drawbacks. You just have to find them. But all in all, it's personal preference. I don't play them very much (i like multi class better then dual class) but it makes for a good change of pace.

Not all Dual combos are awesome, but there are a few good ones. Jan mentioned the Fighter->Cleric which is indeed, a far better kit than a pure Cleric.

The point of Dualclassing is to boost your second class.

Kensai->Mages are ridiculously overpowered( they can wear robes), Ranger->Cleric is not too bad also.

I would play with a dual-class fighter/theif so I could do anything without Yoshimo.

Dundee is absolutely correct ... dual classed fighter-clerics can be really awesome. My best example is, annoying personality aside, Anomen. After you give him the guantlets of DEX, Anomen becomes a really great fighting tank/cleric. Put 5 *'s in warhammer, give him Crom Faeyr, and he'll be kicking tail in no time flat!!!

I think the key is determining the proper level at which to dual. Too low and you don't get enough advantage from the 1st class. Too high and you must wait (seemingly) forever for the 1st class to reactivate. (My guess for anytime you have a fighter as your first class, is that the "best" time to dual would be immediately after upleveling at a level that earned you another weapon proficiency point. I'm thinking somewhere in the 7-9 range.)