A question on making a simple split power supply
I'm planning to make a variable split power supply from a 9V DC
  adapter.


You are going to be disappointed if you expect more than a few mA from the negative supply and also you will need a buck-boost regulator if you want to create 5V to 12V rail from a single 9V adapter.

Same is true for creating -5V to -12V - you need a much more rigorous approach than either a 555 negative voltage generator or a couple of resistors and capacitors.


  How reliable and regulated 9V DC adapters in general?


You can get very reliable ones and can can ones that are really crappy. Best rule here is do some research and pick a reputable supplier.