Landeril: I actually disagree with Iwub here. A Cleric/Mage is a pretty decent character if built right. Especially as Dual-Class. I used a C/M in the BG Series throughout the entire series. While I wasn't as perfect as a Cleric, or as Masterful as a Mage, I could handle most mobs with only a minimal party.
In IWD, I played a C/M, with a Fighter, a F/T, A Thief, and a Cleric as my party members, I even took the F/T out and ended up with a pure mage, and a Ranger into my party ending up with this.
Half-Elven Cleric/Mage
Human Fighter
Halfling Thief
Human Cleric
Elven Mage
Elven Ranger.
With those six it was the first time I beat the game. My second party kept my HE C/M, and Human Fighter, but replaced the others as follows
Elven Mage -> Human Mage
Halfling Thief -> Halfling Thief/Mage
Human Cleric -> Elven Cleric
Elven Ranger -> Dwarf F/C
But again, this is all on your personal playstyle. I'm a heavy magical player, I love mages. I've actually done a party with three mages in IWD2(Third Caster was a Sorcerer).
So I see no issue with you using a C/M, even as your primary, just be sure to build it right mate. One slight mess up in a Dual-Class can screw you over at the worst moment. I learned the hard way with a F/M/T
Our disagreement is probably because of different playstyles. You said in your last post that your a heavy magical player, I much prefer using weapons because weapons are always available, you don't need to rest to replenish them and there are no saving throws to worry about. I tend to use magic that compliments melee so I don't usually hurl fireballs and skull traps in hordes of enemies distracted by summons, I simply use such spells to soften the enemy up.
The other problem is I am not a great fan of the cleric. Yes, they have some good buffs but they are crap at dealing damage and their offensive spells ain't that great either. They have some "save or else" disablers which are crap because A) IWD1 is loaded with undead immune to the effects and B) summons do a much better job at tying up the enemy. The other is flame strike which I don't like simply because the casting time is too long. I don't like having a character who can only buff as later in the game, buffs aren't always needed, even on insane difficulty. Such a character (assuming I don't need any of the situational spells) usually ends up filling his spells slots with healing spells, a function which can be duplicated by a bag of healing potions. To me, this is a waste of a character slot. Barring healing spells, the cleric spells I actually use is less than 10 and thats including the spells I use for certain situations. I am also not a fan of turn undead as more often than not, the turned undead run back to undead you havent turned yet. For these reasons, I swear by multiclass FC's or RC's, I wouldn't multi a cleric with any other class and I definately never keep them single class..
The thing with the FMT and FCM is that they level up a little too slowly but if you are playing on hard or insane diffculty, the extra experience makes them a lot more viable. As long as a FMT focuses on search and open locks, they can still perform well as a primary thief and you have extra weapon proficiencies and spells to boot :)