Building on the hugely successful D&D 3.5, Pathfinder brings so much more to the core game. It takes the relatively simple game system from the Open Game Licence and brings in new ideas, new genres and its own raft of adventure modules.
The rules are largely the same, the core character classes are the same and the core game mechanics all work in the same way. But things to note are that some of the game's weaker rules have been looked at - some things have been down-powered and some things improved. There is a push towards a more natural and consistent application of the rules - but very little really needed to be changed.
The biggest differences are in the list of enhanced character classes now available and the way elements of non-weapon combat are now used. If you're prepared for these differences, and you're used to playing D&D 3 or 3.5, then you'll be quickly up to speed to play this game too.
The game's shared mechanic with D&D means that it is similarly quick to play. Game combat progresses at a suitable pace, and the rules generally bring a logical flow to the way game play carries on. The rule differences do make the game more sensible in places, and have a small impact on the peripherals of the play.
There are many campaign environments available for the game, with a large number of ready-to-play modules available for the game. These environments don't just come with the backgrounds to the world in which they're set, they build up a campaign flavour that can felt across the modules. The various campaign modules I've played feel very different to each other, yet the underlying rule-set is exactly the same.
Being and offshoot of D&D gives the game a strong base upon which to build. The rules fit together tightly and work well for most situations.
There is a wide range of additional material, allowing multiple adventure across many different kinds of world to be enjoyed. There are worlds that resemble the traditional fantasy worlds of the original D&D games, and there are some unusual game scenarios, including pirate adventures int eh Skull and Shackles campaign or demon hunting in the Wrath of the Righteous modules.
There are a huge range of character classes above the original core to choose from and a wonderful array of magic items and new spells that can be used.
Not very much falls apart with the game system. Its biggest weakness is that the core of the game is very weighty. There are a lot of rules and a lot of variations brought in by extension to existing character classes and the introduction of many new ones. Character creation can be a time consuming and complex task, not because it's just hard work (as with Hackmaster) or because it isn't very logical (as with Runequest), but because there are so many things to choose from when selecting the variant paths the character can follow.
Fortunately there is software available that can ease the burden of character creation. The expanding complexity of the game, however, means that computerised help for creating and levelling-up your character is almost a necessity. Any RPG that doesn't make it easy to create and define its characters using a dice, pencils and paper has probably gone too far in the complexity stakes.
Also the huge amount of supplements available can get in the way. When you've got too much paper to read before you start playing you wont get to play the way you really wanted. This is caused by the commercially driven nature of the game, and applies equally (or possibly even more so) to D&D which has the same problem. There is a commercial need to keep selling new rule expansions, environments, classes, etc. through new books or adventure module packs because this keeps money coming into the game publisher's coffers - which helps ensure that these games are kept updated, and new adventures are made available.
The final downside is that if, like me, you've been playing D&D for years, then the switch can come as a bit of a culture shock. There are rules around that you don't use very often and that you know inside out. Some of those rules no longer apply - and just enough to be frustrating at times.
Pathfinder is a really good game. It builds on D&D, irons out a few of the rules weaknesses, and provides a wide range of new places to go and new adventures to have. It's well constructed, and the core material is easy to understand as is largely clear and well laid out. As a game system that is at times complex and pseudo-realistic and at other times simple and fast paced, it delivers well.