With a fast prime you can get great shots in low ambient light. The key is shooting at high iso and getting the exposure right. If you have, say, an XSi, T1i, 40D, 50D, 5D, or better... you can shoot at iso 1600 and still have a relatively clean image.
Bounce flash is over rated. Why? It produces flat lighting and, usually, raccoon eyes. Sometimes the flat light is ok, but if used all the time your images will end up with the same flat light look. Shadows add depth and capture the mood of the scene. By using your spot meter and exposing for the shadows you can ensure that shadow detail is still captured. It's true that focusing at f1.8 and below can be tricky, but you build your skill the more you practice. Inside the house wide aperture is usually better anyway so all the clutter is lost in the background.
I shoot a lot of candid moments at home with our kids and my wife. I do own a 580ex but I rarely use it for these type of shots. I usually don't post them on the web, but if it helps I can dig up a shot or two where use of flash would have ruined the moment.