The cystaid, cystease thing I think qualifies as an FAQ. I was asking it myself about six months back and since then I've seen it brought up at least 4 times.
It works by adjusting the alkalinity of the urine so that's not going to do any great harm to any non-cystitis cat. Tilly has her Cystease sprinkled on the food once every other day. I tend to close her in a room away from Jumpy when I first put it down, she eats a fair amount and leaves the rest. I then let her out the room... sometime she'll return and finish it, sometimes Jumpy will finish it. He's never shown any ill effects.
I figure if she gets at least half the powder into her then it's good enough... so far it's worked very well. No cystitis since we started this regime.
I think Cystease is every bit as good as Cystaid and as you say, a lot cheaper! The vets don't like it as they don't get as much commission from selling it.