One word: boring.
In the beginning, you'll deal with small time suppliers and dealers. You use your cell phone to get a "Felix Lead", which is basically a mission that will then allow you to contact a supplier.The main problem is with these missions, which you'll be doing for much of the time playing Scarface, is that they are either "kill these guys" or "deliver this" missions, and they're not well-made either. For example, you may receive a mission to kill some guys. Instead of simply telling you where they are and telling you to go kill them, you have to do some pretty nonsense stuff, like: first you have to go to a car. When you enter the car, you'll be sent to a location i.e. "gang's hideout", which often is lamely situated in the middle of a street or something. THEN you get the directions to the guys you have to kill. And after you kill them, you have to go someplace else to receive the money and finish the mission. It may be a bit more "realistic", but sure is boring.
After you do a "lead" mission, you get the option to contact a supplier. It will be either a small or large supplier. Small suppliers deal in grams of coke. These can be sold to either small dealers spread around the world or to your fronts. Here comes another nonsense part of the game: if your gang heat(a certain stat) is "cool", small dealers pay $100 for each gram, however, your fronts will normally give you $50 to $90 for a gram, which doesn't make sense at all - dealers want to get a cut at the profits while fronts are owned by you and all profit should go to them. However, you'll often sell these grams to your fronts since a dealer only buys 100g at a time, while a front can buy all you have.
Large suppliers are the great stuff, however. They sell you keys(kgs, kilos) of coke. After you get a storehouse, you can buy keys and store 'em to distribute later. You talk to the supplier, buy the drugs they you choose a car to make a run for your storehouse - gangs and police will try to stop you, but they're not that much of a challenge. Then, you can start a distribution mission, which you go to your fronts and drive over boxes of money normally in their back alleys. This will net you LOADS of money, however it's also worth noting that 1 kilo of coke will give you much more money than 1000g would(yes, you can have more than 1000g of coke in you, and it doesn't turn to kilos if you amount it to that much), which is kinda bizarre.
That said, the main problem is that you'll spend half your game time doing this, and you HAVE to do it, because you can't advance in the game without money, since money is used to buy exotics(cars, henchmen, boats, furniture for your mansion, etc). And then your gang heat will get too high and you'll have to do this more often to pay off your gang heat, or do Enforcer missions, which reduce your gang heat but are as every bit boring as normal Tony missions.
It starts doing right, but the lack of innovations later in the game just kills all the fun in it.