While this isn't an absolutely essential addition to the D&D rules, it's still an important one - other realms have always been a major part of the game, especially at higher levels. And this sourcebook's the first attempt to repackage them for 3rd edition...
It's not bad. This is a DM supplement, so it's a little dry and not as immediately engaging as some of the player books (it's certainly not as easy to skim-read), but it does just what it says on the tin.
On the plus side, there are new spells, new prestige classes, a couple of new items and a selection of new monsters. Plus descriptions of each and every plane.
On the negative side, some parts of the book aren't arranged very clearly - for example, there's no section for items so they're all detailed in different places, the "random destinations" chart is tucked away halfway through the spell list (next to one of the spells that uses it- but not terribly handy when you need it for one of the other spells!) and while there's plenty of guidance on customising the cosmology, I'd rather not see that as one of the things at the very front of the book - how about clearly describing the 'official' version first?
Among the very good stuff, though, are the spells and one or two of the Prestige classes - the roguish plane-hopping Gatecrasher is a personal favourite - as well as impressive creatures like the Inevitables and some of the updated 'classic' AD&D monsters.
As for the planes themselves... the rules are clearly laid out, but sometimes the other details are a little skimpy. While some of them get a good six pages, others just get two or three - and once you've deducted half a page of environment rules (and sometimes a map and/or an encounter table, too...) that's not a lot of space to detail a world. The things that are really missing here, though, are illustrations - some of these strange realms get wonderfully atmospheric pictures, but the vast majority just get text. Key locations on each plane are also briefly mentioned - each one usually gets a paragraph - so while there's not enough information to immediately use, there should certainly be enough to get the DM's mind working.
Oh yes. And you get statistics for Bahamut (the Platinum Dragon, guardian of Celestia) and Tiamat (The five-headed Chromatic Dragon, a resident of the Nine Hells), too. Although hopefully your players won't be stupid enough to fight them!