Warning: Rant level 8.5
This is one of those PrCs I've been looking at, but I really can't get a handle on. I have a really hard time imagining why anyone would take more than 1 or 2 levels of this class (and they would only take it for min/max reasons or Darkvision).
Mainly, it doesn't have a lot of synergy with any other class. The various PrC in Complete Warrior (to take an example) allow you to grow as a character in new directions, but they built upon the abilities you already had. Horizon Walker is so random that it does not seem to do that. Spellcasters immediately cease progressing in any spellcasting ability, so that's out. Even Bards have a good enough spell progression that I wouldn't want to trade it in, let alone give up advancement in Bardic Knowledge and Bardic Song powers. Rogues loose their Sneak Attack progression, and they don't really need the Skill Points.
The only class I can really see taking this PrC is a Fighter whose decided to exchange fighting feats for Skill Ranks and some weird abilities at higher levels. I just can't see it. Why would a Ranger take this? He has to give up his spell progression, favored enemy progression, weapon style progression, good reflex saves and his special abilities (like Woodland Stride, Swift Tracker and Hide in Plain Sight) just for some skill boosts and +1 Att./Dmg. to terrain specific monsters? Dimension Door at will, once every 1d4 rounds is cool ... but would you give up 6 levels of Ranger just for that? You could get an item that allows you to do that 5/day and probably never know the difference.
The other problem I have with this is class - how often do you really use the special abilities? If you're in a volcano that Fire Resistance 20 is handy, but your +30 fly speed in weighless environments is pretty useless. Since most of your skills and abilities are terrain specific, and you can only be in one terrain at a time, most of your abilities are going to be pretty useless most of the time. They aren't flexible like Wildshape or a spell list.
Lastly, for a Horizon Walker with all kinds of Planar abilities, why can't you Plane Shift, or become Ethereal or even Teleport? You have NO travel abilities (except for Dimension Door). A Wizard or Cleric of same level is a much superior "Horizon Walker" than you are.
This PrC has just bugged me ever since I saw it. The Horizon Walker (IMHO) should be - I dunno, have some better abilities that really compliment another class, or are good enough to be their own class. This PrC just seems like something that would always leave me a bit dissappointed.
Sorry about the rant.