If you're a fan of Warhammer Fantasy and L4D, you're going to have an absolute blast with Vermintide.
Several unique characters, several levels that connect via a story of invasion, driving them back and then taking the fight to them.
My favourite character by far is the Bright Wizard, controlling the Winds of Magic and specialising in Fire. So you can blast and blow up enemies with powerful fire spells, or slash them down with your back up sword. You do need to be careful though, as using magic too long can result in you exploding from the build up of power. As such you need you to frequently exhaust the excess heat. Although this can result in you taking quite a bit of damage as well.
I suppose it serves to balance out the Bright Wizard's awesome damage capabilities, and forcing the player to control and manage it more. It's still very fun, and adds quite a bit.
I don't have much experience with the other classes much; although the Pistol and rapier wielding Witch Hunter is also a lot of fun as a medium and close range fighter.
Story and Lobby wise everything starts and ends in the Tavern, and there's even a mission to protect that. In there you have access to different missions, levels of difficulty, the Forge, and Inventory.
Each character has unique items they can use, although through the forge these can be combined to get random new ones, or scrapped for upgrade materials. So if you're lucky on the end of a mission through the dice roll ( which is based on mission performance ) or crafted you can upgrade items to unlock their extra bonuses; if they have any.
The inventory system is rather self explanatory, you can change items based on their attributes and looks which results in some minor character customisation.
In regards to missions, they're fairly linear, with some off shoots which leads to extra mini areas and little treasures, and supplies.
Graphics and performance wise the game does very well. Although there are still some off frame drops here and there, even without a swarm of enemies. Even so it's not much of an issue, although if you have a Midranged or lower graphics card I wouldn't recommend pushing the settings to extreme. I'm sure these will be fixed very soon though.
The main enemy are of course the Skaven, and it wouldn't be like them if there weren't swarms and swarms of them during certain parts. It's good then that the vast majority of them are rather dumb, badly armed and weak. Although if you do happen to run into a unique one such as a Commander, or Gunner you'll need some well placed Headshots to make short work of them. Otherwise you're in for a small world of hurt.
Sadly unlike L4D, other players cannot play the unique Skaven units at this time. They probably might never, considering that they're still very inferior to main Human characters. Sorry Skaven fans.
All in All it's a blast, and even more fun with three other friends. I would gladly recommend it as one of the great Warhammer Fantasy games in quite some time.