I GMed on the high tier with 5 players. This is a difficult scenario combat-wise, and the players have a lot of latitude in what they can do. As the GM, you have to know the manor pretty well to run it, which is different than some. They had a lot of fun roleplaying with the caretaker and warring with most of the rest. A good balance of story, rp, and combat (leaning somewhat toward combat, as most do). Everyone enjoyed it, but they were sweating at certain points!
(Should mention in advance that we played low-tier) This adventure was totally unbalanced, and probably the dullest scenario I've played out of around a hundred of them. Those of my players who happened to bring a good face-character, or a skill monkey, got to play. Otherwise, they were ignored for literally hours while other players described in excruciating detail how they talked to dinner guests.
There are literally two fights, and they're short, pathetically easy and just about pointless. I don't know about your group, but my group was bored to the bone by this one. The only guy who had any fun was the cleric, who did all the talking.
If you like to throw dress-up mystery parties, you might like this scenario. If you're not into that, steer well clear!
I've played all these scenarios as a player, and I own the paper for GM purposes. This is a perfect mini-campaign, just the sort of length I prefer and I wish they would make more of this size. I find full adventure paths to be on the long side, often taking more than a year to complete, and modules to be on the short side. Thornkeep is perfect. You can wade through these short, varied adventures and take a group through several levels in a month or two.
The adventures themselves were interesting and difficult; combat-oriented for the most part. You'll never be forced to endure two-hour session of listening to the bard describe how he's romancing some noble woman before he rolls his D20 at last.
I will say on the downside that the last scenario was weak. GMs should take license with a few of the details to make it play coherently. Also, the dungeons are cramped with larger groups (probably so they could fit it all on a flip-map). I had to declare a pet moratorium to fit people into rooms.
I ran this one as a player, the main tank. I still think of this dungeon a year later. I felt a real challenge here. Very grim and tactical. Several situations that can kill players, possibly this is giving it a lower rating. The only piece of advice I have is: take a tiefling!
Ran it as the GM. I liked it, was a straight fighting crawl with just enough good roleplaying flavor added in. Perfect mix in that regard. I will warn GMs it has some freaky monsters you have to spend some time looking up, and it's pretty hard on the party at level 1-2. But everyone lived and they enjoyed it.