Payday is a four player co-op game that emphasizes teamwork and creativity. The game offers a variety of missions that range in difficulty and length. Maps also offer randomly generated spawning to keep things from getting too stale. You basically pull jobs so you can buy expensive customization items and skills. So you can take on tougher jobs and buy more expensive items and skills...so yeah it's a grind fest.
Success in this game is almost entirely dependent on the cohesion of your squad. If one or more people do not have a mic you will fail. On higher difficulties if one or more people are the same class you will fail. More importantly if there is not a strategic distribution of specials (Ammo pack, Health pack, ECM etc) you will fail. In fact you will have to try really hard to stay alive. It's not a game you can lone-wolf and do well. Unfortunately if you are dependent on random matchmaking you will run into the above issues nearly every game. A good tip is to take hostages in remote areas of the map (they can be freed by the cops) as they serve as extra lives. When you get busted you're teammates can trade them for a get-out-of-jail-free card.
Where it really shines is when you think ahead. You have a dependable squad with complimentary builds and decent chemistry. It's quite rewarding to pull off a heist without the cops showing up. Unfortunately stealth requires a skilled team and some high level skills to be effective. Most of the time your going to be in for a "Horde" type firefight. The combat in the game is fast and furious so make sure you equip aggressively at first.
Equipment unlocks much like Call of Duty as you level up. Higher levels get better guns and armor. Mods and customization items are randomly awarded after missions. Once awarded you will still have to pay for their installation. This is one of my biggest complaints as they are much too expensive. For example putting a flashlight on my assault rifle cost me 120,000 dollars (Roughly 7-8 missions worth of work) and considering missions can take upwards of 15 minutes a piece given the type. It's a long long grind building up your character the way you like it. A entire play session can result in you only being able to afford a new mask color or rifle stock. At one point I had 12 unused skill points as I could not afford buying any of the new abilities I supposedly earned. This game is a time suck!
In conclusion this is a highly addictive game that might earn itself a niche spot on your shelf if you have the friends for it. Just don't try playing it's god awful single player mode or play too often with randoms. Furthermore if graphics are a big issue for you I would not recommend this as it's quite last gen in that department.
As previously stated I would give it a solid 7.5/10 and recommend it to select FPS/Stealth fans.