[Tcoat]

The systems in the Subarus work very well. My wife had had them in two of her cars now. The BSM is one of the best I have seen.
No you should not totally rely on it the same as you shouldn't tailgate because you have antilock brakes or smash into a wall because you have airbags.
When used in conjunction with proper driving practices things like the BSM beat the hell out of little mirrors stuck to your mirrors. A quick glance for the light will tell you if a shoulder check is even worth it and actually keeps your eyes on the road longer.

As far as moronic drivers go at least these features give them a chance to catch their stupidity. If they are not going to look at a very obvious indicator they sure as hell are not going to do a proper shoulder check.