Well, what are the plusses and minuses? On the plus side, he's no spring chicken at age 30, but is experienced. He's got great size, 340 lbs. Fills a need, which is size in the middle (though absent pass rushing ability, I'm not as invested in the necessity of size in the middle as some here but see below). Only cost Dallas a 6th round pick, which Dallas could probably pick up with a compensatory pick, if not even a higher pick.
He's played at both DT and DE, has some versatility. Most importantly, he had 7 sacks his second year with the Giants, as well as 3 sacks a couple of years after that, so he does have a bit of pass rushing ability, apparently. Durable, only missed 6 games over his last 5 full years.
On the negative side, he's never made the Pro Bowl, not even as an alternate, so we're not getting a top player. 30 years old, so he's probably not going to be a long-term piece of the puzzle, though it's always possible he could be. 30 is an age where players do drop off quite often, so he may not be the player he was even a couple of years ago.
All in all, I'd say he should be at the least a good player, maybe even very good. For the draft capital we gave up, I'd have to rate this pick up as about 7 ouy of 10. How he plays will tell if that's about right....