Dempster could easily be worth two wins to Texas through the end of the season over what the Rangers would have gotten from the shattered remains of Roy Oswalt, which are being vacuumed off the Ballpark at Arlington mound as I write. That said, Dempster has been extremely lucky this season in several areas that are not actual skills, from a .245 batting average on balls in play to an extraordinarily high strand rate, and he has the worst ground ball rate of his career.
However, he has two strong offspeed weapons in a mid-80s slider that he throws for strikes (and throws quite often) and a low-80s splitter that functions like a hard changeup with some late tumble to keep hitters from elevating it. He's good enough to start a playoff game for Texas, but I don't think he's the No. 1 starter for which many Rangers fans were clamoring -- even though the team is four games up in the loss column and didn't need to pay that kind of price.
He also talks about Villaneuva and Hendricks, writing that the former boasts "major league potential as a plus defender with doubles power" while also clarifying that he was already blocked by Adrian Beltre and Mike Olt, and calling Hendricks more of an organizatoinal starter who may or may not play as a No. 5 starter at the big league level for the Cubs
I don't know that I'm overly optimistic on Dempster's chances of furnishing a two-win upgrade over Oswalt -- Dempster's at 2.2 fWAR in 104 IP this season, and is moving to a tougher league/ballpark on top of it, and if you assume that he can give the Rangers, say, another 70 innings down the stretch at that same fWAR pace from Chicago, that's about 1.4-1.5 fWAR of value. I realize Oswalt had been a problem for the Rangers, but to get to a two-win upgrade, you'd be talking about Dempster needing to sustain his Cubs pace for the Rangers and Oswalt needing to pitch like a replacement-level arm (or worse) down the stretch. I don't know that I buy that.
I can buy Dempster as at least a one-win upgrade, though, and that helps.