I am a big fan of all three Mangini records, although I would concede that Portnoy is the more enjoyable and fun drummer to listen to. Mangini is a beast from a technical standpoint, but his playing is a bit too clinical. But, despite that, the songwriting since he's been in the band has been mostly great.
I think where DT misses Portnoy the most is promotion. Portnoy was great at giving updates online and getting the fans excited about a new album, tour, etc. It seems like no one in the band right now gives a rip about having an online presence to get the fans excited. Token updates on FB that are likely done by Roadrunner aren't even close to the same thing. Even if Portnoy overstated things, he at least got you excited that, hey, maybe the album was gonna be awesome.
Live, I didn't notice a difference when I saw them twice on the DT12 tour. I am seeing them in 16 days here in St. Louis, so we'll see how that goes.