On the question of putting carbs in the pack, I have heard of triathletes just throwing their bladder away after an event. Of course, they are paying big $, so it's not exactly a dent in their event costs. The pelican bladder I have seems like it would be pretty easy to clean though.
I used to use a hydration pack all the time for long rides. I tried again recently, and carrying the weight was uncomfortable. I have never had much trouble with mine being sweaty, the airflow is pretty good. Someone told me that hydration packs lead to dnf on long rides. He was talking about longer rides than 5 hours though.