I have the Scrubbing Bubbles automatic clean shower spray. It is awesome. You push the button after shower and it leaves a film of some sort that deters / cleans the shower.
No soap scum, no hard water or rust stains, no shower curtain "ick".
I do clean every few weeks with a scrub...Comet, Ajax, whatever I have, just for good measure, b/c it weirds me out to not clean it. But to look, it wouldn't seem to need the scrub.
There are two people here, and a bottle lasts about a month, they are frequently on sale and have coupons, so I usually get them for around $2.50.