When I first brought my rats home, they wouldn't use the bottle. I had to "teach" one of them, by holding the little ball in at the bottom with my finger, and then he'd drink...after a day or so, he just needed me to get it started and hold my finger there in case he needed help again...it took him 4 or 5 days to be able to do it himself consistently. Silly baby rat.
They like the cheapest bottle I've seen though. I bought it as a back up, but they favoured it over the pricier one I bought, so I recently got a second cheapie one. They're the smallest size of just plain "super pet" plastic bottles, I think 2.99, maybe 3.99 each. I have two of them on my 3 rats' cage and change them out every day and they're never even half empty.
I might have left them with bowls except they were always using the water dish as bath water, or they'd tip it over, or they'd just fill it with crap. Always within a couple seconds, so it didn't seem like a reliable long-term solution for my guys.
Just thinking about it though, a bottle sounds a LOT more sanitary...and just more reliable long term. And yah, they really should have more than one water source, just for safety.