Yeah, I've played almost the entire part that I've written so far. Naturally, the group is supposed to gain levels through the adventure, but the prison is the very beginning so it is to be played by characters of the indicated levels.
If you think it's TOUGH, it means that it's doing what it was designed to do: scare, not kill! The prison is - with a few exceptions -
way easier than it seems, combat-wise. First of all, a lot of the encounters are cakewalks for a party of the specified level. Secondly, as soon as the party gets to the lower prison, they have a safe place to rest just minutes away from the dungeon. The monster zone isn't supposed to be completed in one foray.
My party is composed of 3 to 5 characters (attendance is very variable), each one created with 25 point buy, and most of them slightly underequipped for their level. They played the upper prison very cautiously, using darkvision and low-light vision rather than torches, saving spells as much as possible, and talking to all NPCs that could help them. In the lower prison, they entered the monster zone four or five times before finding out everything they needed to know to get out. They were scared witless during the first one or two walks in the zone, and had to retreat (well, actually flee) more than once. After a few encounters with the creatures of darkness, they developed sound tactics to prevent sneak attacks and to avoid detection very effectively (ready actions, never use open light if you can help it, use summoned creatures to draw the monsters away from you). You should also remember the auto-enhancement of all damage-dealing spells. Throughout the entire first part, this works entirely to the players' advantage: no enemy has big boom spells, but the party wizard can rock the house with 12d6 fireballs and shotgun magic missiles.
Up to now, there were three deaths, which sounds fairly bad but all of them were the result of very, very stupid decisions which aren't going to be repeated any time soon (hint: if you are calculating whether your friend can afford to be in the area of a fireball,
remember the auto-enhancement of damage spells). The adventure isn't directly out to get the players, but it looks like it is.
Terror is the point; dead characters aren't scared.
As for adapting the adventure to run off the Prime Material, that should be easy. It has a definite Planescape flavor, but when it comes to actual Planescape elements it doesn't have that many. If you decide to run it, make sure to let me know what happens!