Easy to solo depending on your class and build and which dungeon it is..rogue's/barbarian's are pretty much the easiest to solo with.
That said it's extremely party based though....and it's way more fun in a party as well since it's more than just tank+healer+dps'ers...the dps'ers actually have quite a lot of things they can do besides just doing damage if they spec right. For instance, there are traps and hidden doors and locked doors in loads of of the dungeons, rogue's can learn to pick lock/spot/search/disable device(traps) etc. the best. Sorc's/wiz's etc. that can learn spells can be totally support based and go full buffs with maybe or two damaging spells + go UMD for wands and whatnot. Bard's have song's which are special class-only buffs (probably best in the game), paladin's and cleric's can heal/turn undead/do tons of damage to undead type monster's plus have a few aoe party buffs.
