Just finished my first trail 50K and very pleased with my Nathan TrailMix 7L hydration vest. Bladder was easy to fill (and clean/dry afterwards) and the mouth valve worked much better than both of my friend's Camelback's valves. The front pockets held my 2 soft flasks secure for the whole run. I also did a short training run with regular 500ml water bottles and they did fine also.
While I did not need to use all the storage space during my race, I chose this vest to also use on future all day hiking trips. Plenty of room for extra snacks, rain poncho, flashlight, etc.
Like some others have mentioned, the front straps did loosen while running, but it was simple to reach up and pull the strap to snug it up again. A minor annoyance. I had no issues with chafing or rubbing through my single layer tech shirt.

I ran 8.45 miles with this beat first time out with it and it didn’t bounce and was comfortable even with a tank top on. Nice vest and for my longer hikes has plenty of room for a small lunch and necessary supplies for a day hike.

Not very comfortable even with larger size and like everyone said the straps do not tighten at all

Got this bag for a 100k race. Purchased in July and raced in November, the stitching started to come apart during the and I am now shopping for it's replacement. Because of the short life span I'll be looking at other brands
The bladder hose was long when arrived, I was able to cut it to the right size for me which was nice. The bite piece did eventually leak but replacements were able to be purchased. The pockets were a good size for all the mandatory gear, however I am shopping for at least a 12L to replace it as I expect to need more room in the future.
Overall this has been a great pack for me, but I do not want to get a new one every year so for that reason it gets 4 stars. If you are looking to carry water and a few small items this bag is great. Or of you are gentle on running equipment or should be good. I am neither of these and require more rugged equipment.
I will probably continue to use the bladder, and may get more Nathan bladders as that part of the bag was great. The bite piece is ready to use, and I like the light weight open close mechanism and how easy it was to fill at aid stations.

A really good pack. I've had mine over a year and it has held up well with my day hikes and marathons. I have washed it multiple times (for the pack – delicate cycle, air dry only; and for the hydration bag insert – hand wash in warm soapy water, cool rinse). The pack is light weight and works well with or without the hydration bag insert. The clips and drawstrings are sturdy. I couldn't be anymore satisfied with this brand and product.

Based on the sizing guide, I’m within the “regular” one, so bought it $$$. Done multiple long distance training runs and already finished a few 7:30-8’ pace Half Marathons with it. It works. Has plenty of options for storage (jacket, Gu’s, IV’s, bars, phone, etc.) and 2 liters of liquid is a happy middle ground for the distances I normally do… But, again, even though I’m within the sizing guide; it feels small for my 6’ 200# ‘German’ build. Another annoyance is, like others have mentioned, the chest straps EASILY loosen; not that big of a deal for me as I basically always want them maxed out, but when you’re low on water, it makes it bounce/sway, and that’s annoying (and yes, a quick stitch and/or tie would solve that issue, but again, wearing jackets or not, amount of gear, etc., you want it adjustable, so I just grin and bear it). On that note, I haven’t had any issues with chafing. So again, it works, and I don’t want to hassle with returning it after I’ve used it a half a dozen times. But for the $$$, I didn’t expect these kinds of annoying issues (not breaking points, just annoying; get over it), especially from Nathan; a company that has a long history of running products, so I think a good chunk of the $$$ is in the name.

I have used a few times and it is working well. It replaced one that I have been using for years. Good color scheme and a fair amount of usable pockets, the front pocket fits my cell phone XR with a case. The storage in the rear is not spacious but adequate for essentials only. So far it fits well and it's comfortable to walk, jog and run in. But you need to get used to wearing this type equipment. Works well in cooler weather but in the sub tropical heat and humidity of Hawaii, let's just say hydration is just a trade off, it may make you fairly warm. In the very cool temperatures of Washington or Oregon it is perfect.