I've played two multi-class barbarian/warlocks (mainly warlock), and they're great! You just have to design around the idea that you cannot cast spells (or maintain them) while raging. Since warlocks only cast (usually) 2 spells per short rest, then choose pre-battle buffs with long, non-concentration durations (armour of agathys is perfect in combination with damage resistance from Rage!), and a pre-asskicking fireball!

I've also played a very flavourful vengeance paladin/fiendish chainlock, which I totally loved!