I've been a fan of the Age of Empires series and enjoyed playing Age of Empires III. So, I was quite happy to get a chance to play the Age of Empires III: The WarChiefs expansion. I've just spent a week playing it, and it was a very good continuation of the AoE III experience.
I installed it on my rapidly aging Dell with a 2.4 GHz Pentium 4, and 512 MB of RAM. I received a warning that my system did not meet minimum specs when I ran the expansion but ignored that as I've never been good at following directions. Sure enough, instead of seeing nicely detailed shadows scrolling across the landscape, I saw jagged blocks of darkness patchworking across my screen. My framerate was around 1 fps. But despair not, a quick visit to the options and turning all the graphic goodies to lowest had my framerate up to 30 fps and the game looking great. I later installed it on a powerful machine and it looked fantastic with a good framerate, but it was enjoyable and pretty even on my sub-spec machine.
If you are new to AoE III, then a quick review: You tell your settlers to build structures which allow you to build new units and gain technology improvements. Your settlers gather food, wood and gold from the map in order to build the structures and new units. As you play, you can progress your civilization up through 5 ages of technology. AoE III takes place during the early settlement of North America by the Europeans. It comes with a nice campaign that has a decent amount of historical information without any of the educational software preaching. You can also play skirmishes against and with computer-controlled opponents or online with other human players. As you finish each game, you earn points which can be spent to buy cards. These cards are used in a deck and during play can be used as you progress through the ages.
AoE III: The WarChiefs comes with an entirely new campaign set in two acts. As the name reveals, the new story gives more of the perspective of the Native Americans and allows you to play with new units and leaders from three different civilizations: The Iroquois, the Sioux, and the Aztec. I played through each act in 1 day (roughly 8 hours each day). Overall, I enjoyed the campaign and felt the voice acting was nicely done. My biggest complaint with the entire experience was that early in both acts, I had a great deal of difficulty in winning a map. This meant that I had to restart and try again after losing several times. This was on the normal difficulty setting, and I'm not a new player of these types of games or this series. I was able to defeat both maps easily once I played through, saw all of the objectives, and figured out the way the designer evidently intended for them to be played. Without giving away everything, let me just say that if you are a cautious base-building player who likes to create strength before venturing forth, you are going to have some difficulty... just let go and charge in there.
There are other new gameplay elements as well:
The sound in the expansion is very good quality, with great music. I had one audio glitch, probably due to my sub-min-spec machine, which made the cannon voiced response upon selection sound like the voice-actor was using a cheap microphone. Since all of the other voice work was so high quality, that stood out. It also speaks to the attention to detail that marks the entire series. There are a variety of audio responses for each unit and interesting taunts/expressions from each of the different leaders when controlled by the computer.The native civilizations have access to the Fire Pit structure. When this is built, it allows for villagers to be assigned to it to dance for different benefits for their side. This makes for some interesting early decisions as you decide if you want your unit production to increase or if you want those villagers out gathering resources. 
A Trade Monopoly can be purchased if a player owns more than half of the Trading Posts on the map. This kicks off a 5 minute timer which if it expires before the enemy can destroy a Trading Post will result in a victory. This provides an interesting variant but felt cheap when most of the rest of the game was about military action, not economic. A really nice addition is the Treaty which formalizes a 'no-rushing' rule. It means that 10, 20, 30 or 40 minutes can be designated with nobody attacking during that time. Many new cards are available to be earned and the deck is now 25 cards. Your old decks can be copied and upgraded with the new cards.
The Havok engine is used in the game to create very nice physics for the explosions caused by the cannons. Lots of nice detail in the animations and in the combat action.
I had about seven days of fun playing through each of the new territory maps that are in the expansion in skirmish mode with the computer. While none of them are spectacularly different from the maps that came with the original, it was very nice to have new environments.
In summary, the expansion is what I expect from an expansion: it gives me more of what I liked in the original, and provides nice additions that augment the experience without destroying the spirit. If you liked Age of Empires III, buy the expansion. If you like quality real-time strategy games, then try Age of Empires III and the expansion.
I have 2 children am married, live in Arizona and work too much. I've played games since I was born and have played electronic games since they've existed and am very hopeful for their future.