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Yep, got the 10l one. I use it for anything over 10 miles normally so I can carry drink, emergency snack, layers etc. and for long races. It's pretty solid, build quality is not top notch but it does the job and is comfortable over ultra distances. My jacket fits over the top of it too which is handy as you don't have to stop and undress to put a layer on. You can fit two 500ml soft flasks in the front pockets too if you need to carry more drink.
Have you experienced using the 10l one? Looks like the 15l is pretty new as there are comparatively few reviews. The 10l bag has excellent reviews, will definitely try and take a look in the flesh.