My flash also allowed me to try something new: Manual Mode. It’s scary, it’s not always easy, and it’ll definitely slow you down. But I think my best pictures ever have come in Manual mode, often while using a flash and going for a specific effect. Being able to control aperture, shutter speed, and ISO manually is huge. It often means taking experimental shots to get your lighting right, but after that it makes for better pictures that look more like what you were expecting.
On the other hand, in a place where lighting can vary dramatically from shot to shot (outdoors, shooting into the sky instead of the shade, for example), it can get very frustrating. There, you might be better off in Aperture Priority mode.
I take most of my pictures in Manual mode today. It’s a lot of fun to take the same picture repeatedly while cycling through various f-stops or shutter speeds to see the differences it makes. I’ve learned a lot about photography that way. And once you get the right exposure locked in, it’s a beautiful thing, just as you wanted it to be.
Digital pictures are cheap, and easily disposed of. Don’t be afraid to take test shots. Play with Manual mode, at least, and learn what you can about the technical aspects of photography, and how different settings influence pictures in different ways. Just don’t ask me to manually focus. That way lies complete and utter madness.