Yep, I use one on longer runs so I can stay hydrated.
Originally they were all small rucksacks with a sleeve for the bladder.
In the last 2 or 3 years there the trend is more "vests" rather than "rucksacks" - a lot closer fitting, sitting quite high on the body, no waist strap. Most will still take a bladder, but in the last year or so the trend has been more towards front mounted bottles, or soft flasks.
Problem with bladders is you don't know how much water you have left, and difficult to fill up on the run.
For shorter long runs I still prefer the bladder rather than having the water sloshing about in bottles on my chest ( not tried the soft flasks which are supposed to prevent the sloshing as they collapse as the water levels decrease ). For really long runs where you need to fill up then would go with the bottles for ease of re-filling from streams.
I just find I get out of breath trying to use a bladder. I prefer front mounted but ideally nothing.
Anything less than 3 hours I'd normally be OK with nothing.
The vests are new, but seem pretty dominant now. I was looking at the Scott Jurek one,looked nice. But inov8, soloman etc.. they've all got their own versions and there doesn't seem to be much difference.
As you can tell by the replies it really comes down to personal preference and you can only work that out by experimenting.
I've used a lot of options over the years, from waist packs with side bottles (not stable enough), an OMM pack that I would put water bottles in the side pockets (convenient, but would sometimes fall out), a Decathlon pack with bladder and front chest compartments (cheap and OK, but too constricting across the chest, and bugging on longer runs).
I finally used a Salomon Advanced Skin vest with a bladder for the last ultra I did and was blown away by how comfortable it was. The bladder is easy to fill from inside the pack and has a fold top which makes it straightforward even when you're exhausted.
After 16 hours with it on my back it never once felt heavy or hot and everything I needed was easily accessible. They're not cheap though but compared to the 40€ Decathlon one it made such a difference - I guess the extra money goes on attention to detail.
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