Played Payday: The Heist for 2 1/2 hours now and loving it. The game did exceed my expectations by far. Would buy again.
My observations so far:
The experiences I had is probably tied to the group I played with - the guy I bought the 4-pack from was so nice to invite me and play with his buddies, so we had nice voice communication going on and no trolls/ random internet idiots, that screwed up on intention just for the lulz, in our group.
Unfortunately there is no push-to-talk option and everyone has to mute their mics if you don't want to annoy your team mates (luckily for us, we all seemed to have a mute switch on our headsets/mics).
The game only comes with 6 levels but they are quite long and you will probably fuck up several times, especially on the higher difficulty levels. There are no checkpoints. This means if you screw up, you have to start all over again.
Even though there are only 6 missions, most of it's objectives and enemy spawn points are dynamic. This increases the replayabilty and fun factor by a large margin.
The levels are quite diverse and some took inspiration from movies (prime examples beeing Heat and Die Hard 1)
I'm a mediocre FPS player - I can't really judge the skill of my teammates but I got the top score for 'most kills' / 'most special kills' some times so we seemed to be of equal skill - and we found the normal difficulty setting to be a challenge, on par with 'hard' setting of most other FPS games.
We managed to complete 2 levels on the first try and utterly failed at the third. Then we played another level on hard just to see what it's like and, on three different tries, didn't even made it past the first objective - but at least we made it a little further with each try; changing strategies and learning the mission basics.
Pretty much like you've seen in Left 4 Dead you get (a lot) of trash enemies thrown at you and on occasion one of the 4 different special enemies. Ammo is scarce and you have to make your shots count. Spray 'n pray is no viable option.
The game features a level up system with many different skills to be unlocked through 3 different paths. You unlock all manner of things from new Weapons through passive bonuses and group buffs (increased count of X, more 'XP' - in this game money -, resistance to damage, decreased recoil etc.) to special equipment like ammo and medic bags.
Before each mission starts, you can choose your equipment and bonuses. Beeing level 50 doesn't mean you are god, but instead you are given a broader choice of tools to get the job done.
When you get shot down, your team mates have a couple of seconds to revive you. If they fail to do so, you will be taken out of the game.
The other players can get you back into the game by exchanging a hostage for you.
Civilians and wounded enemies can be ordered to lay on the ground / cuff themselves respectively. You can further secure civilians by cuffing them (you can only carry a certain amount of cuffs).
If you don't watch over civilians they try to make a run for it. If they are cuffed, the police has to reach the cuffed civilians first before they can escape.
You can't drag the civilians around sadly. I believe this was left out by the developers on purpose so it won't get too easy with too many respawn options for the players.
The graphics aren't state of the art but look pretty good IMHO, but then again I'm not much of a graphics whore (and thank god for that). The animations from all characters, players and NPCs alike, look good and smooth. But the feeling from walking and running around can feel a bit clunky at times.
The only real gripe I have so far with this game, besides the constantly active microphone, is the lack of an option to lean around corners/cover. But, again, maybe the developers thought this would make the game too easy and left it out on purpose.
All in all I got the feeling that the game is aimed more towards the hardcore brand of players, especially on the difficutly settings above 'normal' - not unlike Left 4 Dead. I think L4D is really the game that comes closest to this title. If you enjoyed the former, I'm sure you'll also be pleased with this title.