I keep my 3L Camelback in the vented area and it works alright. When the bladder is completely full, you can feel it a little protruding in your back, but after you drink maybe half a litters worth you can't even tell its there anymore.
This is a great day or very light overnight pack. I have used it for about 40 miles so far in the hot Florida weather and the way the pack was designed to fit your back really keeps nice ventilation between the pack and your back. Also caries weight very well.
Installs in seconds and works like a charm. Some have suggested that it increases their gas mileage, but the on-board MPG calculator on my car read the same before and after using the fairing, so I would not say that the difference is all that measurable. One note, I went though one of those automatic rag like car washes and it made it though without a hitch.