The new classes are variant classes as per the suggestions in the DMG on creating variant classes. They are:
Basiran Dancer: bard focused on dancing
Brigand: Rogue that's a bit tougher and less crafty
Gladiator: Fighter specializing in gladatorial combat
Infiltrator: Rogue with a pinch of ranger good at stealth, stalking, and scouting
Shaman: Druid that follows an animal totem
Spellsinger: Sorcerer who casts spells by singing - all spells have only verbal components so it is excellent for multiclass fighter/spellsinger (no arcane spell failure)
The expansion on the skills is really cool, most notably a nice broadening of craft, knowledge, and profession as well as some new uses for old skills. There is also a nice variant speak language system which lets you account for people having different skill levels in different languages (i.e. I speak really poor dwarven, fluent common, and am a master at elven rivalling great poets).
There are over 100 feats. 6 are copied from the splatbooks but the rest are original. A handful are tied to specific regions or races but most are good for general use.
For forgotten realms, I would guess that the following would and wouldn't be useful for you:
Ch1 - Races: Not very useful since the FRCS covers races for you already (30 pgs).
Ch2 - Classes: 100% useful
Ch3 - PRCs: Almost all can be easily adapted to FR. Only two of them are tied so specifically to Kalamar that they might not be easy to use elsewhere.
Ch4 - Skills: 100% useful
Ch5 - Feats: Mostly useful. There are only a handful of regional feats.
Ch6 - Religion: Describes the canons (religious texts) for Kalamar gods. There is a conversion chart for Forgotten Realms so a lot of these books could be ported across to FR. I'm not sure about exactly how well it would mesh, but this is a cool chapter and I think some of it would definitely be of use.
Ch7 - Equipment: 100% useful including some cool herbal, alchemical, and poisonous creations.
Ch8 - Combat: 3 1/2 pages of fighting schools are closely tied to Kalamar and not useful, but the new Kick, Shield Disarm, Throw, and Unbalance combat actions would be fine in any campaign setting.
Ch9 - Adventure: New diseases would work great in FR, but 8 pages of rules on nobility, clergy, and sainthood would probably not work well in FR.
Ch10 - Magic: Rules for clerics channelling positive/negative energy could probably be adapted to FR but would require some tinkering as they are pantheon specific (note the existance of Greyhawk-FR-Kalamar pantheon conversion tables though). Two pages on the divine right of kings would not be a direct fit for FR but might be a cool modification nonetheless.
Ch11 - Spells: Many new spells to use, but this book reprints many of the Forgotten Realms and Defenders of the Faith cleric domains, with the accompanying spells. About 45 spells are reprints out of around 130+ spells total, the rest are new and almost all portable.
Ch12 - Magic Items: 100% useful
Appendices: The feat reference and spell lists include all FR material and are useful in any campaign.
That's about it. Most of the material should be very easy to use in any setting since it's fairly general in nature. Even some of the Kalamar specific parts could be adapted if you wanted to do so. I haven't tried it myself yet but I plan to use it in a homebrew.
