Yeah, and you can take it a step further by setting up a proper rain barrel system off a downspout. It collects way more than a single container, so you can water everything for weeks after a good storm. The plants really do seem to do better with it, maybe because it's softer water without the treatment chemicals. It also just feels good to use, like you're actually working with the weather instead of against it.
But is it really worth turning into a whole project? My gut says you'll just end up with a bucket of mosquito larvae and some weird green algae scum. Most people's gardens need a couple gallons a week, tops. Seems like a lot of hassle to save maybe five bucks a year on your water bill.