When I first started road cycling, I used a simple 2L Camelbak for a few years. I used a saddle bag for the usual stuff and wore a cycling jersey with pockets. The Camelbak didn't interfere with the pockets. I did get some crap about it from "style men" who are all about what you look like, not how you ride. I didn't care about that.

I know there are large Camelbaks with pockets, etc. I don't think those are a good idea. If you're going to need it on the bike, it has to be in your jersey pocket. The saddle bag is for stuff you'd have to stop to mess with anyway, like punctures and clothing changes, food to refill jersey pockets.

I never thought a Camelbak was hot, quite the contrary. On hot days, I'd stuff it full of ice and add water. It'd be cold on my back. One of the issues with bottles is that the contents get hot so quickly. Insulated bottles aren't that great because they're hard to squeeze and hold less water. A Camelbak is so much better in those respects, plus it's 2 liters! I never noticed that it was heavy or caused any back discomfort.

The main reason I used a Camelbak is because I felt so much safer using it than a bottle. It's always convenient to take a suck from a hose. That convenience leads to better hydration. The other thing is that I'd carry liquid food in a 24 oz. or liter water bottle, so I could be carrying 3 liters if I wanted, only water in the Camelbak.

After a few years of doing group endurance rides, I got more comfortable with bottles and that's what I use now on most rides. I still use that Camelback on long difficult rides. I'm noticeably faster well hydrated and also faster the less I have to stop. Riding the one-day STP or similar, I only need to stop every 50-60 miles for a refill in summer. There's a 10,000' century in the North Cascades with the only water at the halfway point. Camelbak.