Grok is a good hero due to his built-in and inexpensive teleportation ability. Add to that the demon's mana, and you get enough casts to take down quite strong neutral creatures using Cerberuses, for example. Good starting logistics as well – it doesn't take long to upgrade the Path of War skill, and, as mentioned before, you can get war machines; demons are simply made for ballistas (or ballistas are made for them :D). I've always respected logistics-focused heroes, and Grok, in my opinion, is the best among them, but only because of his teleportation ability.