I've technically talked about MotP before, but that was mainly in response to a podcast review that Red Jason and myself thought was fairly lackluster. The topic came up a couple nights ago, so I figured I'd talk about it again and do the job proper.
Manual of the Planes is the big 4th Edition book on the planes, and weighing in at 159 pages it covers exploring the planes, devotes a chapter to each individual plane, has new monsters, and wraps up with a chapter for additional player options. I would consider this book almost entirely for a DM if it werent for the strange inclusion of paragon paths, but I'll get to that later, and by later I mean last.
The first chapter gets you started on the planes and how they can fit into your campaign. It goes into the types of planes, planar traits, portals, planar customization, rituals that can get you there, and my personal favorite: vehicles. Flying astral ships and spelljammers are listed, among other things. Not wanting to make things easy, there are more than a few planar hazards for DMs to throw at their players to spice things up a bit, and even Sigil gets a few pages alongside the Far Realm (yay) and Plane of Dreams (yawn).
All in all, this chapter serves as a good primer for how you might use them in your games, how to get your players there, and how to challenge them on the way.
One of the biggest changes to 4th Edition aside from giving the boot to half-orcs and gnomes are the planes. Unlike the cosmologies of yester-edition, the new layout is condensed yet functional. While the planes have been reduced to a whopping four (if you aren't counting the world), the design team at Wizards is geared towards making sure that the content both is useful and usable, and I feel like that despite having less than half the former amount I can get a lot more out of them than I could before.
Mainly I think its the lack of incredibly deadly environments. It used to be that many planes, especially the inner ones, were immediately deadly if you didnt have the proper protection. By protection I mean a basic spell that left you impervious to the plane's harmful nature, which is really just another way of providing a minimum level prerequisite on adventuring there, not that there was anything to do in most of them, which brings me to my next point.
Once you cast your arbitrary spell to let you completely ignore what made the plane dangerous in the first place, you probably wondered why you bothered. Many planes had little reason to exist aside from symmetry: each alignment called for its own plane, and each inner plane could combine into a para-elemental plane. Most of them were endless expanses of the same, or just minor variations of the Material Plane.
For example, The Elemental Plane of Fire was just a bunch of fire, and the Plane of Vacuum was predictably a plane of absolutely nothing. Even a lot of the outer planes were fairly mundane. Bytopia was a realm of industrious people that had a parallel world above itself, and the Outlands was your basic wilderness zone that had an impossibly high-but-not-really spire in the center. I guess it also stretched on forever, if thats any consolation.
Even the more exotic looking planes ended up being useless. Mechanus was an infinite expanse of gears with stuff built on them, but there wasnt really any need to actually go there since the hundreds of modrons maintained the place pretty well without any interference, even when the formians showed up in 3rd Edition.
Comparitively, the only planes that seem anything like the normal world are the Feywild and Shadowfell, though this makes sense considering that they are respectively light and dark echoes of the world.
The Elemental Chaos is situated beneath the world and is he kind of mess you would expect if you mashed all the old elemental planes together. Unlike the old elemental planes, there is plenty of stable space that you can walk around on without exploding. At least immediately. The Astral Sea is the other major player, existing as a kind of heavely realm above the world that you can sail across (preferrably in some kind of planar vessel) in search of godly realms so you can beat them up and take their stuff.
Aside from the utility, I also like the new setup because it actually feels more fantastic. It moves away with the assumption that underground there is a mantle and core, and that above the world is the cold vacuum of space. Its more inline with real-world mythologies in that if you went underground deep enough, you could feasible reach the Elemental Chaos. Reminds me of the part in God of War where you literally climb out of hell.
Now, I'm a big Planescape buff but I tended to avoid most of the planes and often just had the players do urban stuff in Sigil because the Great Wheel seemed fairly artificial and the planes I really wanted to use had to wait until my party was high enough level to cast the right spell to get in and adventure there. That delayed if not outright killed the stories I wanted to tell unless I allowed them to make characters of the necessary level to cast whatever immunity spells they needed.
If you do like the new cosmology, then you're in for a treat because this book does a very good job of covering the bases. Each plane gets their own 15 or so pages of fame, detailing how to get there, who lives there, important locations, and in some cases new crunch like the soul rot disease. The Astral Sea gets the lion's share of content at 26 pages, but then it goes into more extensive detail about numerous astral dominions bobbing about. You get a healthy mix of solid adventure ideas and rough concepts to work with, and for the first time in years I've been excited about planning planar adventures beyond using them merely as interesting-looking backdrops.
While the book is mostly DM oriented the inclusion of the paragon paths makes it just a little out of place. I dont think the book has any kind of "identity problem", but since DDI has been out for awhile it makes me wonder why they didnt just feature it on Dragon. My theory is that they wanted to include not only options in the form of paragon paths, but also additional rituals to get players there, and even some new thematic magic items. Think about it, how many people would bitch if they put the rituals in Dragon?
A LOT. They would complain that Wizards is "forcing" them to get a DDI account so they could get material that should have been in there all along, blah blah blah you know the routine.
Getting back to the actual paragon paths for a moment, I really like most of them, especially considering that when it came to specialized content in 3rd Edition it was almost universally useless if you werent adhering to the theme. You could take a planar prestige class, but couldnt really use it if you werent out planehopping. The paragon paths in this book that are pretty thematic (like the gatecrasher) work just fine even if you prefer to stay close to home.
The short of it is that I would get this book if you want to expand your campaign beyond the natural world and/or round out your selection of monsters, paragon paths, and/or magic items.