Gameplay-wise it's like the tabletop version with tweaks to make it more video-game-y. If you had ever played Blood Bowl or seen its gameplay, you know what you are getting into.
I haven't progressed enough into the game to critique the storyline yet. For much of the game you are fighting other warbands over a fistful of wyrdstone to make quota to ship to your sponsor, and the quota increases with warband rank. After a few in-game days, you get a story mission which you can do to progress the story. For Imperial mercenary, the first story mission had you go and seal off the sewer system to keep the skaven out so you can capture the quay for good, thus allowing you to ship wyrdstone to your sponsor more efficiently. However, being the greedy git that he is, he ends up increasing the quota so that leads to another story mission a couple days later, where you are going to have to go even deeper and closer to The Pit to get more wyrdstone. But first, you need to go visit the Temple of Sigmar and get blessed before making the trip (though the local holy maidens have something to say about that).
By the way, you can get your wyrdstone from online matches, and you get to ship the wyrdstone you got from online matches to your sponsors to make quota. However, be warned, if you miss quota once, your sponsor throws a hissy fit and the next quota will be double the previous one plus interest. If you miss quota four times, you are locked out of the campaign missions, but you can still use the warband to play online matches.
To ensure that you do not get completely screwed over by your loss, the veteran skill points can be carried forward to your next playthrough to help make the next warband more manageable to organise.
Edit: Before I forget, you are going to run into daemons as you progress far enough. Daemons come in four flavours, as usual, and they are neutral. By neutral I mean they are hostile to everyone, the Cult of the Possessed included. You will only get one for each mission.
Being daemons, they are really difficult to deal with. Best thing to do is to lure them to your enemies and take the opportunity to scatter to the eight winds and loot all the wyrdstone while your enemies are busy trying to not die to a daemon.
Also, the level design is fantastic.
Another thing to mention is that your henchmen need not necessarily have to level up to gain skill points. They can get skill points by simply performing well enough. Even if you lose missions, you can theoretically 'win' by managing to get away with wyrdstone, and your henchmen can still gain exp and skill points from a lost. Also, your henchmen's permadeath chance is also determined by RNG so it's possible for a henchman to get knocked out only to manage to crawl back to base needing treatment or, occasionally, be perfectly fine. He might still need to replace his gear, however.
However, there's also that chance of permanent injury, which are always debilitating. And they add up. Good luck with that.