On the one hand, yes, he is useful. He's pretty good in a fight, damn near indestructible (I have all the DLCs except Zeta), and since he's small I can shoot over the top of him in combat without him getting in the way. Plus he can find things for me.
But, and this is a big but, the SOB keeps trying to get me killed. Here's the latest example:
Going to get the dish for Three Dog. So I get through the metro tunnels and everything without too much trouble, except for that one bit with the turret, and come out near the museum. And as soon as I climb the steps up from the subway I'm taking fire from the super mutants. Then Dogmeat charges. And returns bringing back a fair few super mutants with him, including a master with a supersledge, whom Dogmeat then went toe-to-toe with.
Net result? My endurance 5 level 6(now 7) character was forced to dive into the super mutant trenches to avoid being mulched by minigun fire, Dogmeat kept drawing in more and more of them, and my guy is getting dangerously low on stimpacks.
On the one hand, he does get my guy out of some nasty situations. On the other, he often causes them. I have got to find a better follower, maybe one with a gun.