I'm A Banana
Potassium-Rich
Looked interesting, but also alignment-restrictive.
Interestingly, it was Planescape that made me finally OK with the alignment system of D&D and the Great Wheel cosmology. It went to great lengths to make sure that, for instance, just because you were Lawful Good didn't mean you weren't an antagonist, and it actually made you think about what that meant -- to believe that Order and Altruism were the best way for you to live, and perhaps for everyone to live.
I suspect the former.
Would have to be. PF can't release an "official" Planescape supplement, because Planescape belongs to WotC. BUT, I am under the impression that Pathfinder hired a Planescape guru to helm their planar stuff, so it might be awesome anyway.

We've seen no alignment problems yet in our one session, and while some of the races (modrons) seem horribly restrictive, and others don't make sense (githzerai can't be lawful, so no monks?) I'm pretty sure it's the DM writing 2e flavor onto Planescape mechanics. (Eg house rules, not a setting problem. There were no monks in 2e.)
Alignment-wise, you might want to look at it this way:
Modrons have to be Lawful because they are made of law. Order is the foundation for their existence. Philosophy makes up their biology (such as it is). The ordered thoughts of the multiverse are what give them flesh and blood. For a modron not to be lawful would be like for you not to be Carbon-based. It's literally an alien way of being.
Githzerai can't be lawful, because the race's culture values personal freedom as one of the highest ideals you can aspire to. To be self-determined, to be a slave to no one, to be free to think and act as you please...these are core Githzerai values. One could imagine a Lawful githzerai like one might imagine a Good Mind Flayer. Okay, maybe it could happen, but it's running counter to the entire core value of the rest of the race.
That said, when I run PS4e, I'm not going to have the characters pick an alignment. I'm going to have them act, and then place an alignment on their actions over time. Thus, alignment will be an emergent feature of gameplay and of your character's philosophy, rather than a core choice that you make.
We started at 1st-level (is that even appropriate for the setting) and so far have fought hobgoblins and gnolls. It doesn't really feel like Planescape yet.
1st level is appropriate for the setting -- one of Planescape's big additions was the creation of places for low-level adventurers and "normal folks" to live a daily life on the planes. But upper planes and Sigil itself tend to be the stomping grounds -- planes that aren't threats in and of themselves.
And I dunno why your DM is pitching hobogoblins and gnolls at you. Sounds like maybe you could get a better feel for the setting if he tossed you out onto the great wheel and engaged you with some demons or something, at least for your first adventure or two. But maybe it's in the mix, coming up.
