Got the HC book, but haven't played it yet. Heard many good things about it though.
One of the advantages is that it's very flexible - not surprising, considering that the rules are supposed to be used from several millenia B.C. to the 1300s or thereabouts. While there are the seperate books with armylists (and points values), you are generally supposed to just have a look at your era and area of interest, pick some units, give them applicable rules (normal book has basic profiles for light/medium/heavy infantry and cavalry etc., as well as a range of special rules to give them). So Spartan hoplites would be heavy infantry (with some moral boosting special rules) in their period, while Scottish gallowglass would have similar rules in theirs. Scenarios are mostly supposed to be made yourself from what I remember, using historical situations - Arminius' Germanic tribes ambushing Varus' Roman legions, Vikings invading King Alfred's territories etc. Forces don't have to be, or indeed aren't supposed to be, balanced: it will take some effort to win if you're outnumbered, outclassed or both, but that's the idea of a historical simulation.
With the supplement books, typical Xpts armylists can be made for a wide variety of factions, for a more standard gaming experience.
The actual rules themselves are very interesting: they truly make you the general, commanding your troops.. and every now and then, you orders are misunderstood, ignored or not heard at all! Tactics, intelligent manoeuvring etc are important, although some lousy dicerolls can of course still cost you the game. Before comitting to buy, you can have a look at some articles on the WG website to get an impression of the game (i.e. http://www.warlordgames.com/10274/hail- ... at-system/
). It is a much more gentlemanly approach than you would expect from e.g. WHFB, and is ill-suited for very competitive play I think.
As you can imagine, the rules can easily be adapted for all manner of circumstances: you can make Mordor Orcs 'medium infantry' with some morale penalty (examples are included for units that are freshly levied) and put them against much more elite Númenóreans. When sticking to historical scenarios, the main downside is of course that you need two armies of roughly the same point in time, which means that finding the right opponent can be tricky! Best to see what is played in the area and adapt to that, or simply collect two opposing armies yourself.
Shouldn't be too difficult finding opponents in the Nottingham area, seeing how the Warlord Games HQ is situated there...