This game was nominated by Origin for "Best Roleplaying game in 2007".
I played it at Kublacon 2007 and had a great time.
Okay. So what's this game all about?
Think of pulp action stories.
If that doesn't ring a bell, think if movies like INDIANA JONES, JOURNEY TO THE CENTER OF THE EARTH, ROCKETEER, BIG TROUBLE IN LITTLE CHINA, DOC SAVAGE, DICK TRACY, KING KONG, JURASSIC PARK, AT THE EARTH'S CORE, THE LAND THAT TIME FORGOT, LOST HORIZON, SHE, THE SHADOW, LAND OF THE LOST, TALES OF THE GOLD MONKEY, YOUNG INDIANA JONES and THE TIME MACHINE. Or, think of authors like Edgar Rice Burroughs, Jules Verne, H.P. Lovecraft, Philip Jose Farmer, H.G. Wells and Sir Arthur Conan Doyle.
You get the idea.
Action is meant to be fast and furious (you can use special dice that quicken the game), characters are always bigger than life, the villains are meant to also be bigger than life and extra evil (think black and white morality here which means lots of Nazis), the unbelievable becomes believable, science and magic/occult cross paths, dinosaurs walk the Hollow Earth with other types of long past creatures, history can be used or made into an alternative world (i.e. setting is typically early 1900s), various areas still remain unexplored (and are therefore ripe for holding the unknown) and, while Hollow Earth is the main setting, there's enough information here to do something different or even have adventures set in the surface world.
What about characters? Think of fun archetypes from the movies (some here are listed in the core book): Indiana Jones, a cowboy, a big game hunter (who is bored with lions and wants to kill dinosaurs), a curious female reporter (in a man's world and out to prove she's the best), a snooty know it all professor, a dying rich industrialist (looking for the fountain of youth to cure him of his cancer or other disease), a peaceful missionary, a mad scientist, the imperiled actress, the fortune hunter, etc. All have special reasons for desiring to explore Hollow Earth.
I would say the characters are balanced enough, especially if the GM doesn't make it all combat. Most of the characters start not with similar skill levels, of course, but with similar enough ability point distribution (and you can only go so high or low when picking your stats), so, when I played it, even the weakest characters had a fighting chance (though this was based more on surviving than wiping out a more powerful enemy).
Last on characters, there's a good spread of advantages and disadvantages in this game, as well as enough skills and sub specialties to make it interesting for a long term campaign.
Three special highlights of the game come to mind:
1. FLAWS: a flaw is something you play up in the game, like being overconfident in your field or stubborn to the point of it working against you or being a danger magnet (i.e. you attract bad things or go out seeking them). When you play this properly, you get a style point (see number 2).
2. STYLE POINTS: style points can absorb damage, increase your skill ability, sometimes affect the story, give you more of a chance of success in combat or be up to the GM for other matters. One basically gets it for playing their flaws properly, pursuing your motivation, being in character and/or adding to the quality of the game.
3. CHANCE DICE: when the chips are down and you're about to die or get into a very bad situation, you can call on more dice to increase your chances. The only problem is that the number of dice you select makes the obstacle harder to accomplish. As the chance is typically 5-10%, you really only want to use this if you are truly desperate.
OTHER NOTES: this game lets you take your average roll as a default to rolling. Think of it like "taking 10/20" in D.20/D&D. Another thing, similar to D.20/D&D, is that you can use your soft attributes (like intelligence and dexterity, per D&D) in combat with the right talent/advantage that lets you. That's a nice touch for the smart or agile types.
What about knowing the setting? There's some chapters on understanding The Hollow Earth as well as what was taking place in the USA and other countries (focused mostly on 1936 but with some basic overviews back to the very late 1800s). There are special resources in Hollow Earth and there are several organizations on the surface world who want to get them. The big problem is that Hollow Earth is very hard to get to and even harder to leave.
The core book also starts you with NPC stats (villains and allies), a basic adventure, equipment (gotta love the "Tommy" submachine gun), and, of course, a bestiary (yes, it has a T-rex).
Overall, this is a superb game for getting into the spirit of pulp. It does use Hollow Earth as its setting but it implies/lists other settings, and, with all the supplements coming out, GMs can run their games in other settings.