I finished Warband just this morning and I have to say that the rules are great, they cover pretty much every aspect of what we want in the Garnia World campaign with one glaring exception- there are no rules for non-human characters. This makes sense because it was written as a historical RPG with fantasy/horror elements, but Garnia World is chock full of other races that HAVE traditionally been PC races; ie. Dwarves, Elves, Half-Orcs, etc.
Now, I know we're planning on starting the 43 AD/Warband dual campaign soon, and having it ultimately lead as a gateway to Garnia World; just as my Norse B/X campaign will ultimately do if it ever comes back online. I suspect I'll have to wait for September or October for that, after Lance's work schedule pretty much grinds to a halt at the State Fairgrounds. Lee Ann is already antsy for some RPG action, but her 12 hour shifts and near constant "on call" status as the new RN at the ER have kept her from making games for like two months now. Besides, I think it's probably better that her husband and her daughters get to see her, despite the fact that it's kind of killed my home game.
Anyway, I wandered pretty far off point with that tangent. 43 AD and it's Warband supplement give us, when combined, the ability to make both Roman and Celtic characters; a way for GMs to design entire campaign areas in either the Roman or Celtic style, fairly quickly and easily; a magic system that is pretty easy to use and rather open ended; a bunch of Celtic monsters, because the game takes place in Britain, although seeing the design style I think it would be pretty easy to import D&D monsters over or create anything. Warband also gives us mass combat rules for both huge combats with 10,000+ combatants and smaller scale skirmishes.
The real pro of the system is that it's pretty rules-lite, the con of the system is that it's less "heroic" than D&D, more realistic in a lot of ways. Character advancement is, essentially, religion based, and the points you receive are, more or less, based on the monetary worth of the sacrifice that you make at the Temple or Altar of the God you are sacrificing to, if your character is a Roman. Celts can make either sacrifices of items of worth or enemies of honor. You may only try and improve one ability per game session, and it doesn't always work. So, in play, I fear character advancement may be damnably slow, the character you start with, you had best get used to, because he's going to be roughly the same for a long while. On the other hand, improving stats and skills isn't everything, Roman Legionaries can get promoted to various higher ranks or than can be decorated for valor. Celtic characters can achieve higher status through gaining honor in various ways.
The only real complaint I have is the disparity in possible social classes between the Roman characters and the Celtic ones. All of the Roman characters start out as simple Legionaries, there is no chance of being an officer or from an important family in Rome, or even an important provincial family. You are almost to a man poor, sometimes freedmen, occasionally vicious criminals that were just never caught. The Celts on the other hand, are all Heroes of Honor level 3, but at the GM's discretion can be brought up to as high as Honor 7 Warchief. I guess that would settle the question of who is going to be party leader though.
I am also making the assumption here that when I run into any difficulties with this system in trying to convert to the eventual Garnia World setting, that I'll be able to fall back on the older, free version of the rules, that were much more fantasy, although still somewhat based in a Romanesque campaign world; that actually might be helpful for filling out the Roman space in Garnia World. Now I fear I'll have to read those rules too.