Many folks yearn for fast 2.8 lenses, and they're legitimate, worthwhile investment for those who can afford it and need it for their assignments.
Most of us can't really afford such lenses without going through some serious budgeting. Even when we do buy such a nice piece of equipment, most users fail to realize that fast lens does have tradeoffs as well, expecially when it comes to depth-of-field and max shutter speed issue.
For most casual users, investing on a flash gun is a much better decision, in my opinion. A flash gun allows your basic kit lens to function well in low-light and fast-moving subjects regardless of lighting conditions.
If you're shooting a group of folks in low-light, using a wide aperture (f/4 or larger) usually won't let you get a sharp image beyond the person your camera focused on. You'll most probably need f/6.3 to f/11 in most group shots - something even your fast lens won't be able to cope with in terms of shutter speed. Even high ISO won't help much, not to mention the loss of detail due to noise.
A flash will allow you to keep a high shutter speed (usually above 1/100) while keeping the aperture opening small (f/5.6 or smaller).
ETTL systems calculates exposure pretty accurately and if you have the experience, you can always manually mix and match your flash-cam settings.
Here are some examples of a flash's advantage.
Camera: Canon EOS10D
Flash: Sunpak PowerZoom 4000AF (not ETTL compatible - manual only).
Trigger: Optical hotshoe trigger.
Settings: 1/200 @ F/10 ISO 200. Flash set at 1/16 power @ 28mm
Note that all the photos below are shot with the shutter/aperture/ISO settings above except for the ambient shot (set # 3)
Set 1: Off cam
Set 2: No flash