My Experience
I ran this module for a table of five players at tier 5-6 - it was my first time running a tier 5-6 scenario. The characters were a witch, alchemist, two fighters, and a gunslinger. (No divine magic or positive energy, you say? Uh-oh...) The physical atmosphere was at a gaming store, so somewhat cramped and noisy.
Players enjoyed it, and did whatever they could to avoid the haunts. The final combat almost ended in a TPK, and would have if not for a musket master gunslinger critical hit which (effectively) one-shotted the bag bad on round 4 of the fight. I don't begrudge the gunslinger that - with half the party having succumbed to save-or-suck, it was, perhaps, the one way out they had left.
The Good
If you're one of the GMs who think that PFS Scenarios are a bit too combat-heavy, then this scenario will work nicely for you. There are three combats, plus an optional one, but compared to the rest of the encounters, it doesn't feel like combat is a huge portion of the scenario. Two of the combats that are there are challenging without being overwhelming, largely - but there is definitely a chance for a character death or a TPK if the rolls don't go in the players' favour. The Haunt mechanic is used to good effect, and the author has gone out of his way to make the haunts not only interesting, but also relevant to the scenario storyline. While it would have been interesting for my players to have a cleric or other positive-energy person, I almost feel that they would have missed out by neutralizing the haunts - few of them were so bad that the lack of a divine character was felt.
The Bad
I spotted a half-dozen typos, one of which is in a faction mission (Osirion). There's a lot of exploration of largely-empty rooms. I expect that more could have been done to flesh out the appearance of these rooms, but at the expense of the word count, so I understand why this was limited. One of the Haunts had the potential to be disastrous on a failed save, which might have changed the outcome completely. A great map which is just a little too wide to fit on a standard flip-mat is frustrating if you like to draw stuff out by hand.
Conclusions
This one's about atmosphere. Play it at home or in a quieter store - the aura of menace is really important to building the suspense. If other Pathfinder Society scenarios are Raiders of the Lost Ark, this one is The Grudge. Pace it well, build suspense, and freak your players out at the right moment and everyone will have a better time.
Overall Feegle Rating: Awesome, with a side of typos.
For suggestions on how GMs can improve it, see my post in the Discussion Thread