I used both a bladder system and carried a collapsible water bottle as well.
I recommend the Osprey - I have been using 2L version for several years now, no leaking issues. It has a nice wide mouth for filling and a number of other features that I've come to appreciate in addition to the easy-fill mouth: a stiff back that helps when you refill and return it to a full pack, the hose attaches and detaches easily when refilling, the magnet, and there is a genius hook in my Osprey pack that slips easily into the top of the bladder clip so the bladder doesn't sink into the pocket as it empties.
I haven't had any problems with leaking, but I do agree you have to be diligent about refilling on longer days. I like the ease of just taking quick sips without having to reach for a bottle. I think
@Robo has developed an interesting system, but I'm not sure how well that might work for me (filled water bottles bouncing on already large breasts? don't think so, ha!). As I noted, I also use a collapsible water bottle, 500ml with a wide mouth, that I clip to my belt - I fill it with an electrolyte mix in the morning and sip that the first couple of hours until it's done, then roll it up and slip it into a pocket. On most days the combined 2.5L is enough, but I try to stay vigilant about how much I'm drinking and especially if it's lot I refill the bladder mid-day. It's definitely heavy at first!
One issue with the bladder system is that you have to be vigilant about cleaning it - not necessarily while on your Camino, but periodically and especially after long sustained use. Of course water bottles need to be cleaned periodically too, especially since I'm putting a drink mix in mine. Osprey makes a nice system for cleaning and drying all parts of the bladder system - the drinking nipple can especially accumulate some nasty black stuff from coming in contact with your mouth (yuck!) and the tubing is harder to keep clean than a water bottle, so make sure you clean it periodically.
The Osprey was originally recommended at my local REI store (outdoor equipment store for those not in America) by a clerk who used it on his Caminos, but they were out of the 2L size, so I tried to order it online and Amazon did one of those switheroos where you search for a specific brand but instead some other similar item pops up. I didn't notice it wasn't Osprey until after it arrived. I used the fake one for a while, but forgot it at a hotel after two weeks of hiking in Italy where I had hung it up in the bathroom to dry it out the night before my flight. The replacement Osprey I bought when I returned home was so far superior to the fake one, I could really appreciate the difference (and understand the price difference ). I have stuck with Osprey ever since.