Gregg has looked great in the couple of times I've seen him against the Tigers. He got them out of a big jam.
As far as keeping it up all season, well just think of how many pitchers ended up with an era under 2 last year that pitched 80 innings: 4. And over 70 with under 2 era: 10.
At some point a lot of these current low era pitchers will have one or two blow ups, and their era will end up over 2, towards 3. (this means that their era with you will likely be over 3, depending how long you keep them) To me anyway, it's hard to see when these are coming and dangerous to pick them off the wire, since you know regression to the mean is likely to occur.
But I do anyway and I guess that's the "skill" of fantasy baseball. Catching trends and not flukes. I've already got burned by a couple.
"If I have seen further it is by standing on the shoulders of giants."