I strongly recommend you play this game if you love turn based strategy games, RPG games, or are a sucker for a good story. Easily one of the best strategy games I have ever played (and I have played a hell of a lot!) - don't be scared of UPlay and Conflux (more on this later).
The game is at its core a Turn Based Strategy game, but also has strong RPG elements - levelling up, skill points, hero inventory, reputation points, faction variations etc.
I would strongly suggest watching some YouTube videos to see how the game plays out.
Briefly speaking you have a game map where you move your heroes around, collect various items/dynasty weapons for your hero's inventory as well as wood, ore, crystal, gold and fight neutrals. There are also various structures you can interact with which confer various bonuses, or grant units for your army etc.
On this map there are various towns and forts which you fight to capture, and once captured you build up to allow you to recruit more units, higher level units or get bonuses for your battles.
Battles involve moving stacks of units around on a grid, destroying enemy stacks in a variety of ways. There are a large variety of units in the game, roughly 7 types per faction - each with an upgraded version so really 14 per faction. Units often have special abilities, like being able to puppet enemy units to do your bidding, or charge through stacks slaughtering ally and enemy alike. All the factions have very different units so it never gets boring.
In addition to units being moved around, your heroes can also use various abilities each turn, you can unlock more by spending skill points as you level up. There are loads of hero skills split in to Might types or Magic types, and these are futher classified into many sub-groups, too many to list here, but for instance for the Magic branch you have separate skill trees for Fire, Water, Light, Dark, Earth, Air, Prime (not all are accessible for each faction)
Hero Dynasty weapons can also grant more abilities, as can items - many items come in sets (Diablo style) and wearing the full set can give you some very nice bonuses.
There are Five factions in the main game (Haven, Inferno, Sanctuary, Stronghold, Necropolis), and a Sixth faction (Dungeon) in the expansion Shades of Darkness - which I also strongly recommend you get! Each faction has completely different units, different available skill trees, faction specific abilities, reputation abilities, different game play and look very different too.
Sound and music in the game is awesome, the battle sound track is stuck in my head.
Graphics are amazing too; the detail on the maps, on the units, structures, flora is great. Even the main menu has switchable themes - all of which are great (personal favourite is the dungeon one).
The loading screens have some really nice drawings too, I had the Haven one as a wallpaper for some time.
Units have really nice animations, and even have idle animations - Dungeon's Assassin starts spinning his daggers above his head like helicopter rotor blades and gets lifted off his feet and then drops down headfirst (all an animation - has no effect on gameplay..!), Vampires start playing rock music and using their swords as an electric guitar - really shocked me the first time it happened -"where is that rock music coming from..?"
The storyline is amazing - I was eager to learn more just after the intro video!
The game is split into many different campaigns - each campaign has up to 4 missions. The core game has one tutorial campaign, and then one campaign for each of the five core factions. The expansions add four more campaigns.
The story is great but as the core campaigns all run simultaneously in the story chronology, it is possible to play one mission in one campaign and then ruin the story for yourself as you now know the outcome of another mission for another faction that you haven't played yet.
I would recommend you play the Custom games and Campaign missions in the following order:
Heart of Nightmares (Custom game accessed via the Custom game option in the main menu!), Prologue 1, Prologue 2, Stronghold 1, Sanctuary 1, Haven 1, Inferno 1, Necropolis 1, Stronghold 2, Sanctuary 2, Haven 2, Necropolis 2, Sanctuary 3, Stronghold 3, Inferno 2, Necropolis 3, Inferno 3, Sanctuary 4, Stronghold 4, Haven 3, Necropolis 4, Haven 4, Inferno 4, Epilogues, Pirates of the Savage Sea Campaign, Danse Macabre Campaign, Dungeon Campaign, Necropolis Campaign 2.
Maybe copy-paste that list into a text file somewhere and delete missions as you complete them!
The only issue some people have, is with UPlay. Upon installing the game via Steam, you may have issues launching the game. Just install the latest Uplay off the Ubisoft site, see this link: http://steamcommunity.com/app/48220/discussions/0/45350245398257355/
Another issue is that fighting neutrals can get repetitive as they are often so weak. Turn on Quick Battle in the options menu, and if you have a decent army, the battle will be resolved for you automatically.
Gameplay tips - The Reinforcements skill is the key to victory in most scenarios early on- use it on stacks that are going to attack / get attacked and you will never have any losses from your main army.
If you have other issues, just ask in the Steam Forum for the game, there are plenty of people that can help there!
I'll say it again, I strongly recommend you play this game! Or you can just wait for Heroes of Might and Magic VII which is currently in development - can vote on factions to appear in the game etc on the developer's website!