Did you see the web broadcast of the drummer auditions? Mangini's was the first up and he was amazing. From a technical standpoint, I think DT chose the right drummer. Now, the question remains about the quality of the new material and the integration of that with Portnoy's material in a live setting.
Overall, I think DT does well when there is a balance between speed/thrash and the melodic pieces. The "palet cleanser" between dinner courses. Too much of either and this band becomes like too many other groups out there.
I think they need to abandon the slow stuff though. Stick with uptempo and melodic mid-tempo. They haven't done any "slow" stuff that has grabbed my attention since
Falling Into Infinity (where the slow stuff was actually the best stuff on the album--
Hollow Years, Take Away My Pain, and Anna Lee... even Speak to Me which was recorded during those sessions but didn't make it onto the album was pretty damn good!)
From a lyrical standpoint I don't think they've ever fully recovered from Kevin Moore's departure after the Awake album. He was a great lyricist and I think he inadvertantly challenged the other guys in the band to be better songwriters as their material was better when he was in the band as well.