I recently had a request to make an FAQ. Here it is. I will probably update this over time.
Is mining still profitable? Yes, but only barely (as of 8/4/18)
How much is a good price for an Antminer S9? The best place to by an S9 is at Bitmain.com, and you should pay about $625 for the miner and PSU combined. You will not have to wait until a batch is up for order. Don’t buy it from eBay or Newegg, you will be paying way too much.
How cheap does my electricity have to be to mine profitably? At this point, I’d say that if your power costs more than 17 cents per kilowatt-hour, then it isn’t really worth your time.
How much airflow do I need for my miners? You should need about 300 cubic feet of air per minute per miner (300 cfm per miner). It is better to go over that amount than under though. If you buy a fan, it should tell you how much cfm it blows.
Why not just put the miners outside and put a covering over them? Outside, miners are much more exposed, and even if you put a big shade thing over it. When outside, there is always a certain unpredictability. A raccoon or other wild animal could harm them, a windy rainstorm could come through and ruin them. My recommendation: Do not put them outside.
What type of Antminer should I buy? S9, L3+, D3, or A3? Do not buy a D3. They are barely even profitable, and basically a waste of money. Currently, A3s are making the most, but they are at the point where that could change suddenly. S9s and L3+s are good investments, and it is your opinion which you should get.
Why use a PDU (Power Distribution Unit)? Why not just wire up a whole bunch of plugs? First of all, PDUs make it much easier to put a whole bunch of miners side by side. A PDU also acts as a surge protector, so if an electrical surge flows through the wires, it will ruin your $200 PDU instead of your $1000-$2000 miners.
What do you do with the coins you mine? I take 30% of the coins I mine and hodl them for the foreseeable future, and reinvest the rest, or spend them on other things. For example, I am typing this on a computer paid for entirely with the coins my miners mined.
What mining pool do you use? Slushpool.com. I am a huge fan of Slushpool, and they make it really easy for beginners, and their fee is low.