I'm 45 right now, have all 5 of them, and the only companion I use is Doc. For levelling it's great, I never have to stop and heal up, gold elites 3-4 levels above me are cake, most of my levelling is done in zones 2-3 levels above me and I have no problem with doc, I tried others such as the tank Scorpio, but she just doesnt have the longevity, having to heal after every pull, and dying to many gold elites even after gearing her up with blues and purples. And the great thing about doc, gearing is easy, when I get a slight upgrade, I put my old piece on him, so he is decked out with orange, blue, and purple gear and heals like a champ. Soloing is easy with him in his healing stance.