BALANCE:
The class is quite balanced, as long as the Cavalier is effectively fighting on mount. Once he is unable to ride, for example in a dungeon based adventure or indoor scene, the Cavalier is left more or less as a Featless Fighter (given the fact that he is likely to have spent most of his feats to qualify for the PrCl which therefore are mounted feats). It makes for a better villain who meets the PCs on his own ground than for a PC, generally.
FEATURES:
Most of the new features are quite tasteless bonuses to Ride and Lance or Sword attacks while mounted, or movement facilies. They are all going to be useful, and being a Cavalier is the only way to get these focuses, but they are definitely not too much exciting.
FLAVOR/RP:
The class seems to represent quite an archetype of jousting knight, with a honor bound to a kingdom, a lord or noble family or similar authority. It can be thought of as a western counterpart for the Samurai, with the code of conduct left for the player or the DM to define; hence, while the overall concept is simple and solid, there is still space for campaign customization. This is not necessarily a good property for prestige classes, which originally were supposed to be very specific "careers", but on the other hand it increments the usability of the Cavalier to more characters.
REQUIREMENTS:
The Lawful alignment requirement is very in line with the idea of this PrCl.
The high BAB required makes it implicit that the PrCl is for elite warriors, as well as the high number of feats: particularly, four of five feats imply that the character is already expert in fighting on
mount, which makes it clear what all Cavaliers have in common.
Skills needed make very sense, but it's good that the rank required is not too high, and don't penalize too much characters which must take them as cross-class.
An equipment requirement is almost always a bad mistake: exceptions should be rare and should regard the process of getting into an organization. This class is not the case, since which group the Cavalier is going to be a part of, if any, is left open.
DESIGN:
Unfortunately, this PrCl has several little incomprehensible design weaknesses.
Handle Animal should be definitely a class skill, since it is a prerequisite and a Cavalier can't be in any way less facilitated in training his mount than a normal Fighter is. The entry about Knowledge(Nobility) and Burst of Speed are fine after the errata, but the Full Mounted Attack ability needs a real explanation or it doesn't make sense at all. Furthermore, some class features have an irregular level progression for no apparent reason.
A little more attention when editing this PrCl would have made a much better result.
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OVERALL: 5