Just finished it. According to my saves it took me 4 and a half hours in two sittings (started it last night) so the claim that it's the longest DLC to date seems accurate. Granted I was taking my time, looking around the maps for all pick-ups (yet somehow missed the spare part for that batarian engineer), read all the new codex entries and took time speaking with Aria after the missions so I'm sure one could easily cut an hour off of that.
Overall I'd say it was very good, but falls short of being the best DLC. As with the main game, there are a lot of wasted opportunities. The ending I thought was a little sudden and somewhat shallow. I won't get into spoilers but I was very disappointed with the way they handled the reason why Nyreen couldn't come back to the Normandy with you. Also, not getting Omega as a hub world after completing the DLC feels like a *huge* wasted opportunity. Even if it was nothing more than a few extra scan quests and an upgrade shop. On the postive side, they're certainly continuing Leviathan's trend of borrowing certain things from Multiplayer, which adds a nice amount of variety to the combat.
One thing that surprised me though was that they appear to have made a new elcor asset. It may just be a re-texture, but I swear the facial animations were different.