I finally got around to playing and completing the Trespasser DLC (Between raiding with my guild on WoW, working a 9 to 5 job, and juggling the new Forza and Gears of War releases, I haven't had much time to even think about DA), and I rather liked it. The humor was what I would expect (which isn't a flaw by any means), and I liked the way they effectively set up the premise for DA4.
I chuckled at Cassandra thinking that the Inquisitor was proposing to her. Some day, Cassandra. Some day.
Overall, I would give it a 7 or 8. Main problems I had were the health sponge enemies and relatively short length of the DLC (I managed to start and finish it in less than 5 hours, and that's not rushing through it all), and I wasn't particularly fond of the end choices.
I will say this though: If DA4 does take place in Tevinter like Bioware has been saying (and like has been suggested by the ending to Trespasser), then Dorian had ***damn better show up. Also that the Inquisitor had better return as an important NPC with what remains of the Inquisition. They built him/her up as way too important to Solas to just ignore in DA4.
Also, in my game, Cassandra is the only party member to remain in the Inquisition between the main game and the beginning of Trespasser. Is this romance related, or does she just not leave? She's never been made Divine in my saves and she never rebuilds the Seekers, so I can't imagine many other places she could go to after Corypheus is defeated, so her sticking around makes sense. And I'm sure the Inquisitor doesn't mind her staying.
I'm probably going to do a de-facto for-sure completionist canon run through the game and all DLC again. Vivienne was Divine in the save I used to play through Trespasser, and I've always favored Leliana as Divine. How does Leliana's role change in the DLC if she is Divine? Because I noticed that Vivienne was a selectable party member in the DLC, despite being the Divine. Not sure if bug or intentional, because I don't think the Pope would pick up a magic staff to fight Qunari.