I love running with a camelpak...in fact I dont know how fellow runners can do without something similar for longer runs. Once I get over 10 miles I need to drink something so a Camelpak is the best way to help me keep going. I tend to fill the pouch with an electrolyte or an energy drink.
What I find so useful about the Camelpak is that I can drink when I feel like it. This sounds obvious and a triviality but in a long running race when I am not using the backpack I find it difficult to drink what I want and when due to the lack of reliability of distances between aid stations and also the products they use. The longer I have been doing events the more I want to have my own products and consume them at my own pace.
The other benefit I have found is that I can run with my iPhone. I listen to podcasts or audio books whilst running so I put on my current book and run with Bluetooth headphones. Also I can put on the mapmyrun app and it will tell me my pace and distance every mile...very useful. Strava claims to have this functionality but it does not work much to my frustration.
One downside of a drinks backpack is that you make a lot of sound! If I am running and talking to someone whilst running I make a lot of noise with all the liquid sloshing around...
Final tip: if you use a camelpak make sure you keep you bladder in the deep freeze between runs. This will stop it from getting the annoying build up of black fungus over time. However much you wash it after each use I have found it to be unavoidable.