DMH said:
How does it stack up against FFG's Planes and Portals?
I see I need to add to my planar product review project so I don't have to re-type these sorts of answers all the time.
To riff off Kanegrundar, I love Portals & Planes like a fat kid loves Little Debbies. I wrote a whole campaign around one of its concepts.
But I also wrote a campaign arc around an idea in MotP, so they both have a lot of great campaign ideas.
Manual of the Planes has more individual planes and more cosmology building advice. And it is the definitive resource on the "core cosmology" (the great wheel, etc.)
Portals and Planes has more on portals, and more ideas on building campaigns around planar ideas. And it has some great alteratives to the core cosmology.
Unlike some other third party planar products, Portals & Planes doesn't plug into the generic planar format that are now in the core rules. Instead, it has a variety of planar traits that can be combined to produce unique and interesting worlds. But it shouldn't take too much effort to use them together.
I hope that breifly explains the differences. I think, so long as you are flexible about the shape of the core cosmology, you can use them together. I do.