When I started in photography, I used to hate the pictures that I took when I had to use the flash because of the lack of available light. I thought at that time that people used flash only to supplement light so that a picture could be taken (I now know that it is only one of the many reasons people use flash!). The reason I did not like to use the flash was that the pictures I took tend to have portions too bright while other portions too dark. Basically, I did not understand the ONE most important aspect of photography... LIGHT!
Because I didn't like to use the flash, I started to buy fast lenses... the faster the better! Fast lenses have their pros (if not why would anyone pay so much for one), but I also started to learn that using flash in photography also had advantages. The key about flash photography was to learn how to control the light that you have around you (ambient) and the light from the flash, and how you balance that out in your picture.
So... I began to read up about lighting, and flash photography. One technology that is in most camera systems today is the TTL (Through The Lens) technology, and if you are using TTL, it is also good to know how TTL works (I'm the sort of guy who needs to know from ground up how something works). Have a read at Moose Peterson's article on The TTL Flash System.