[Dirty Meg]

'- A multiclass cleric/mage sucks only if you don't know the spell systems well. Ditto sorceror. Either one is kind of a tough choice for your first game.' - Feral

Sorcerers are great but only if you know the spell system inside out.

Mage/clerics suuuuuuuuuuck.
Clerics are the worst spell casters of all the spell casting classes. They make up for it by being pretty good in a fight (good HP, can wear any armour). If you combine the cleric with a mage they become a liability in a fight as well as a mediocre spellcaster.
Spellcasters are weak at low levels, but are really powerful at high levels. Multiclass them and you take twice as long to get at the high level spells.
Mages and clerics have no skills in common. Mages need good int (to learn spells) and good dex (to compensate for inability to wear armour and low HP) clerics need good wisdom. To excel in three statistics you will need to reroll your scores for an hour before playing.
Mage/cleric multiclasses are no use in a fight, either with ranged weapons (they can only use slings) or with h2h weapons (with poor AC and low HP they will die very easily). They are only useful as spellcasters, at which they are worse than a fully fledged mage or sorcerer.