So, we have a book intended primarily for players to experience the planes. What do we get?
One of the cruddiest books that WotC has ever put out, that's what.
Allow me to explain...it's not ALL bad. The new weapons and magic items are nice. There are new monsters, many of which are intended to transport players through the planes. Some of the prestige classes are interesting...the Ardent Dilettante itself is VERY creative, and makes sense for what it's supposed to do. Also, the substitution levels are neat; the idea is that you can trade out a certain level of a class for an alternate version with different benefits. It's a very nice idea, and will probably be used again. After all, what rogue wouldn't want to switch out a +1 bonus to Reflex saves vs. traps to be able to sense gates and teleports within 30 feet?
However, there are things that are unpleasant to downright stupid. For example...we see the aasimar and tiefling, AGAIN. This is their third appearance (in Forgotten Realms Campaign Setting and the 3.5 Monster Manual), and this really doesn't add anything. At least the Bariaur has been altered, and is different...but still, reprints get annoying. Also, the feats, in general, are horrendously boring. I can't see any reason to take any of the feats presented.
And, last but not least, you have the Planar Touchstones. I'll admit, it was a nice idea to give people a reason to go plane-hopping. Personally, I'd think being able to fight new and different monsters and get different sorts of treasure would be reason enough, but hey. However...the idea that going to a place gives you a nifty power is kind of, um, silly. Not only that, but they waste 33 pages on this. They make more sense as web enhancements than as a list of stuff that will most likely not get used. Those 33 pages could have been used for more prestige classes, more feats, more monsters, more items, or, heck, even more description on planar cities, and would be more all-around useful. This is the what absolutely kills this book for me.
I'm not...quite...sorry I bought this, but if I could clip out the 33 pages of the touchstones and get my money back, I would. 2.5 stars, rounding up out of misplaced generosity.