So I'm playing a Paladin, and eventually I plan to multiclass Champion of Torm, but I have a question about that. Once I multiclass, should I stop levelling up the Plaladin class, or no?

I like paladin spellcasting in NWN1. (In NWN2, you only get half caster levels, which sucks.) Paladins have some pretty nice spells, like Greater Magic Weapon, Holy Sword, divine favour and Bless Weapon. Plus plenty of utility spells like Bless,

But it really depends on the kind of module/campaign you're playing. In the OC, weapons and magic items are relatively low power, so you do benefit from getting to cast Paladin buffing spells. In Hordes of the Underdark, though, you don't really need them since you'll be drowning in magic items that do the same or better. For custom modules/online worlds it depends.

A build with only limited paladin levels can be perfectly effective, but it always makes me wonder why I selected the paladin class in the first place. Unless, of course, it's something like a sorcerer/paladin type warrior-mage. (Not a particularly effective character, mind you, but fun to play.)

Yeah, Paladin Spells are not bad, certainly nicer than Ranger spells, but IMO more often than not, you have plentiful magic items. Even in SoU, a Paladin can get a Holy Avenger relatively early (making Holy Sword, GMW worthless).

Then it is what are you giving up: feats, and stats you could move from wisdom to more combat oriented stats.

I often play with a Paladin Splash( 3 levels). It gives me access to Holy Avengers, Fear Immunity, Disease Immunity, and Remove Disease Feat (Which clears lowered stats from multiple effects, not just disease).

It's a pretty good splash class for that. I despise the fear effects that makes my character run away.