At first I was hesitant, but I can confidently say that I love Grunt. He’s adorable! As a tank-bred Krogan who was woken up to humans and other aliens surrounding him, and the will to only fight, Grunt was technically mature for his age. He knew he served a purpose, and he knew he needed to fight. Shepard, using his/her great negotiation abilities to calm Grunt down, vowed to help Grunt figure out his purpose, and eventually find his clan.
In Mass Effect 3, he truly proves himself as a powerful Krogan, and his motivations and power were put to the right purpose. When Grunt comes up and out of the cave covered in Rachni blood roaring and fighting with all his might, I got chills down my spine, and shed a few tears. Even playing the game several times through, that scene still affects me.
As a squad member: I didn’t use Grunt much, but if I felt like I missed Wrex, I’d use him to temporarily fill the void.