I bought this hydration pack for marathon training. I also bought the Osprey Viper 4 and the CamelBak Octane XCT to compare. Those last two were returned.
Overall, I took a risk on the vest hybrid after swearing by CamelBak packs for almost 20 years. It paid off. I love it. Mountain Hardwear did a great job with this one. It fits 100oz of water, my phone, and some nutrition, but could fit a midweight shell with no issue.
Now, the tl;dr part...
With any hydration pack, you're going to feel restriction somewhere. The biggest difficulty in deciding between the XCT and Fluid was whether to restrict the waist or the chest/shoulders. It was pretty close for me, but I went with the Fluid because overall it was less restricting. Moving the upper body is much less important in running than moving the lower body, so restricting the waist is less desirable. I rated these on a restriction scale from unhindered movement (0) to completely throwing off my entire running form (10). This is completely subjective.
Fluid 6: Shoulder restriction 3, torso 1, hips 0. Fits like a vest, snugly against the upper back, and allows for free movement from about midback down. Since it's an upper back pack, there isn't the mid-low back sweat accumulation, which allows for a nice breeze. In 90 degrees, I didn't feel especially clammy on my back as I usually do with packs.
I was worried about pinch-points because of the thick shoulder straps, but didn't get any on my 12 mile run.
The little elastic bands on the shoulder strap are good for arranging your hose where you want it.
The elastic pocket on the front can fit my beefy iPhone with Otterbox Defender case securely.
The actual cargo area is open to the zipper and has an elastic-rimmed sleeve for the bladder. I could probably fit a tube and bike pump and tools in if I needed, too, plus a shell in the elastic bands on the back. There are two side pockets, as well, but these are hard to reach on the run. Despite this, the whole pack can be cinched down with two-point shoulder strap connections and those elastic band on the back. There was almost zero sloshing.
As the bladder drained, a quick pull up on the shoulder straps down near the hips tightened it back up well. It's also very adjustable across the chest with two straps and 15 different chest strap positions (yes, I counted).
CamelBak XCT: Shoulder restriction 1, torso 3, hips 2. Very lightweight. 100 oz, which is good. The weight of the water was distributed near the mid-back, but lower than the Fluid. There was restriction of both shoulders and hips, though the hip straps were well-fitting. If I was looking to double for a biking pack, this might be it (though I'd probably go with the Octane 18 for the extra cargo). There was little cargo room for anything larger than my fist on the back pocket, but the hip pockets were bigger than any other pack's I've seen. A very close second place.
Viper 4: Shoulder restriction 3, torso 3, hips 1. Don't bother. Bulky, unadjustable strap system across the neck and shoulders. (I'm 6'3", jacket size 42L). Seems to have extra weight and too much rigidity, so it doesn't cinch down against my body. Waist straps are meager and uncomfortable. LidLock is useless for running (it's for helmets). No cargo room. Front pockets aren't that great. Would be a decent road biking pack, but not for running.
Bottom line: Fluid 6 is a well-designed, low-cost option for distance running.
What Is Your Gear Style:
Was this a gift?:
Bottom Line Yes, I would recommend this to a friend