although hard, alkaline water will produce a cloudy star-san solution from the instant you mix it, but it stays effective.
the other nice thing is that when it does lose its ability to sanitize, you can add some straight star-san back to it until you hit the correct pH range...basically re-charging it with less star-san than a brand new bucket of solution.
I think the star-san nay-sayers have never actually used it. they don't see that for $9 (thats my LHBS price for the small 9oz bottle) you get so much freakin sanitizer its insane.
for the hassle of having to rinse 'cheap' bleach sanitizer with pre-boiled water...its worth it to stick with star-san.