With a second run, on heart of fury difficulty, even if you play normally without exploits and cheap tricks, i think you get enough XP to get max level, ergo obtain all spell slots that your class and max (30) level allows. The original at least (icewind dale 1), allowed a MULTICLASS character to hit max level on BOTH classes! While BG makes DUAL-CLASSED characters kinky and badass, icewind dale does the same on MULTIz.

Here, Mulitz rulez!

Just to adding on to this post.

If you're playing on insane difficulty with a 6 character party, each character should end the game with over 5600000XP. With this in mind, bards hit level 30 in the ice caves in HOW and single classes fighters hit max level sometime in TOTLM. Multi class F/C R/C and F/Ds can still get their top level spells but F/Ms still fall short at ending the game with 2 level 8 spells. Triple class mages will only have 7th level spells.

But yeah, considering KiNgBrAdLeY7 stated he did a HOF run through with the same party, you will get some crazy XP. With his party though, I would never go though normal difficuly with it as the mages highest casting level will be too low. I would toss out the C/M and put in a specialist wizard instead, preferably a nercomancer. The "evil" aligned divine spells blow major donkey balls too so their loss won't be missed. I'd also make the druid a multi as some druid spells really go well with a multi class, mainly thorn spray. *sigh*, if only IWD1 druids had frost fingers too.
Thing is though, there aren't a lot of decent divine spells as it is and the extra ones you do get from this set up are useless. Animal rage is a liabily if the character goes berserk, the cause wounds spells take too long to cast (better off just swinging a mace) and blasphemy only works on good creatures which you won't be fighting any off.

I'd rather have another character that has the same useful spells another has.

But of course. You in the end will decide things, and what you want to play on. But keep in mind, that 2 clerics, and especially in those combos (cleric mage and cleric ranger), are nothing to scoff at. They are a formidable power. Even those two characters alone can solo through the game almost effortlessly (am testing that and working on it as we speak).

Don't get me wrong, I'm not doubting you, I'm also a huge fan of multi-classing clerics. In fact, you just have to look a little further up in this thread to see what I think of single class clerics. The R\C is just plain awesome, no doubts there. The only thing I don't like about MCing a C/M is that mage spells progress quite a bit slower than divine spells and the XP hit for multiclassing is noticable. This isnt a problem if your C/M is a secondary caster but In your party, it seems the bard is your primary spell slinger. My beef with that is they just don't get as many castings as a mage, no level 9 spells and only 1 level 8 spell. Not sure how fast the C/M can get mage levels in HOF mode but before HOF, they are definately gimped if they're your primary caster.

My favorite use for the bard is to pair them with a diviner. Let the bard handle the summons or haste castings before combat, sing or pitch in during combat, no bard function goes to waste this way :). Diviners can then handle every other spell and has more to cast during combat.

Just take an all-Dwarf party.

Sources:Gimli, Bruenor Battlehammer, Gotrek, Thorin Oakenshield

Ooh... Dwarf sounds strong. Especially one of those fighter/clerics... Iconic class.

Never played in HOF mode, but this thread makes me want to fire up Icewind Dale!

Yeah I was kind of convincing myself to have a run on HoF mode with my saved game, can't wait to finish the download!

I can show you my party and I have little problems with them, even though I play on insane on my first playthrough (now I am at the severed hand).

1.) Human paladin, main tank
2.) half elf cleric/ranger, second tank
3.) half elf bard
4.) gnome fighter/thief (A multi thief has more than enough points to take care of traps, locks and stealth, no need for a pure thief)
5.) human fighter level 3 -> Mage
6.) human fighter level 3 -> druid (druids are great to stop enemies and prevent them from casting)

I used a different alignment and different melee profiency for each of them so there is always somebody who can use the items I may find.

Don´t forget to give everyone a ranged weapon and lots of ammo. Range wins!

I think with 2 pure classes, 2 multi, 2 dual and each one a different alignment this is a balanced party.

Mad3's party is kind of similar to the one I think is optimized, some comments:

Human Paladin - Check
Half Elf Cleric/Ranger - Works quite well, though I think taking Human Ranger and then switch to Cleric at level 3 is better.
Half Elf Bard - Check (but place him at bottom =p)
Gnome Fighter/Thief - If you make him Elf then he's the best archer you can get.
Fighter 3 -> Mage - Is good, alternatively you could also go for Thief 12 -> Mage if you want additional thief skills (then you can learn different ones with each, use one for traps and one for lockpicking for example).
Fighter 3 -> Druid - Is good, might want to go for Fighter 6 -> Druid, though for additional survivability.

about the thief->mage: You need only 1 multi thief, he got more points than you need. Level 12 is much too late for dual class, you want a mage early in the game.

about the clerich/ranger: I choose multi because I wanted more attacs and better hit chance. As written before, clerics do not have many super impressive high level spells.

You also want a druid early, entengle alone will save you often, summons are nice and don´t forget the long lasting AOE damage that keeps enemies from casting.

What if I don't include a thief and just go with
Paladin & Fighter (for tankage)
Fighter (archer)
Cleric (healer & undead manager)
Mage (to id items & add a little damage)

The idea is xp adds up faster with five instead of six. Would I regret not having a thief or could I get by ok like in NW Nights?

I'm pretty sure you would regret choosing that party composition. IWD is certainly doable without a thief, but it would be pretty effing annoying, especially early on in the game. I would strongly suggest replacing your melee fighter with a multiclassed fighter/thief(preferably a halfling, despite the strength penalty).

Can you even beat Trials of Luremaster without a thief? There were plenty of doors I had to unlock and there didn't seem to be any way around them.

Not to mention how annoying traps can become in some parts...

Yeah you don't need a pure thief, but at least a multiclassed Fighter/Thief (you can make him your archer!).

Pure Clerics and Mages will probably make you suffer on the HP side too much. Clerics have a lot of combat potential too but to fully use it you need to combine them with Ranger or Fighter. Mages should at least be Fighter 3 first so they can at least take one hit from all sources. If you take Human Race then you can easily switch to Mage after Fighter 3 so you will hardly lose out on exp.

Though if you just want a mage for "id items & add a little damage" then you could just go for Bard. Much better choice. With bard you won't even need the identify spell and you can buff with your songs endlessly. Not to mention that you can uses pickpocket so your half-thief doesn't need to learn it.
You will miss out on half the spell slots, but usually, I personally never really needed them as damage spells are fairly useless and some buffs and debuffs are important but you only need them for harder battles and after those you will have to rest anyway.

And yes the game is very well playable with 5 characters too.

TotL can be played without a thief, even solo fighter is possible.