Ditto on the ziplocks.
Remember running around with a water balloon only to have it bust in your hand? A condom is alot thinner. It would hold more water, but would seem to bust alot easier when filled.
Try filling a condom from a water source other than a running faucet or a tap. You need to get a little pressure going to fill it up. What if there is no running water?
If you have a decent sized water source you can drag/swish the condom around several times to fill it up, but then you would need both hands to support it and carry it around once it is filled. What if you need to set it down for a minute to take a piss? Yeah, I'm hung like that so I need both hands.
The condom suggestion came from the SAS survival manual. I'm guessing that this was based off of stuff they did during the 60's-80's in SAS whereas ziplocks are a more recent type of thing. I only had the fold-top sandwich bags as a kid.
I'm not shooting down Lofty's kit or his knowledge, it is good stuff but I use it as a base and adapt it to my anticipated situation and needs. I would rather carry a lighter with me than a few matches.
My suggestion would be test it out first (as with all your gear) before you rely on it. Go to your local park/campground for a few hours/weekend and give it a shot.