I have a Camelbak I use for longer charity rides and centuries now. It's huge (3 liters!) but it feels like a seat belt when I'm wearing it -- feels like I'm strapped in, secure. If you like that feeling then you'll be ok. I also used it as a commuting bag before I bought panniers last year.
Here's a lesson many other Camelbak users might agree with. Don't put anything other than water in there, unless you enjoy cleaning the inside bag. I used to put a Gatorade mix in there, but I spent lots of time getting after it with brushes and cleaners at the kitchen sink. On the road, this must be a major inconvenience.
Ken