Fields of Battle is an excellent rules set for mass combat. It's rules-light without sacrificing complexity or interest, just like you would expect from a Troll Lord Games product. It's based on the Castles & Crusades rules set. It's very scalable, letting you fight battles with dozens of combatants just as easily as with thousands. In my experience, combat with fields of battle is very fun, and manages to be fast without feeling like you're giving up tactical thinking. It includes quick, compelling rules for pitched battle, skirmishes, siege warfare, aerial warfare, and naval warfare. It includes cool and reasonable rules that keep your PCs in the fight without making them useless or overpowered. It also comes with three battle scenarios and a 'monster manual' of troop type stats. I'd definitely recommend purchasing this.
However, there are two flaws that keep this product from five stars. The first is that you have to be familiar with Castles & Crusades to use it. Now, don't worry – C&C is very quick to learn, and almost all the rules you'll need are in the FREE quick start guide. However, there are some important rules that are not present in the quickstart guide. What's the range category on a longbow? How long does a druid's Entangle spell last? These things aren't in the quickstart guide. You can pretty easily handwave this stuff, but its absence is obnoxious, and frankly, you shouldn't even have to delve into a second pdf to look up these answers.
The second flaw is the absence of any system to determine how fair a battle will be. This is all well and good if you want to re-enact the battle of Crécy (totally doable with this rules set), but from my perspective, this is a gross oversight. I'm a GM, and I use this system as part of my ongoing campaign. When my PCs march to war, I want to be able to throw a battle at them that will be challenging enough to make them feel heroic, and maybe even kill them if they make poor decisions, but ultimately be winnable if they act smart. Fields of Battle lacks any sort of system to help me plan that. If you want to use this at your table, you'd better have a friend who will enjoy playing a few rounds of Fields of Battle with you so you can develop a feel for difficulty levels before you break it out for your regular group.
All in all, an excellent product, and one I definitely recommend purchasing. Only a few faults keep it from being truly stellar.