I find Wyll very useful as a warlock, though I'd like Larian to adjust his stats to actually fit his backstory. His dexterity is too low to be a successful fencer, yet he is training the tiefling kids in swordfighting when you meet him. With high dexterity and the powers from his patron he could have passed as a fighter, which is what his story implies.
As a warlock I give him the devil's sigth and agonizing blast. Since he can position himself with misty step, he does fairly well just blasting away. What I find very useful is his imp summon, since it has unlimited uses of invisibility. This means at the start of combat you have an invisible summon you can send after an enemy archer or spellcaster.
And well, after a few playthroughs I just send the imp to scout the areas, since familiars can activate waypoints. Easiest trip to Underdark.
